The model structure for potential trip planner app like 'tripit.com' or 'travelerbuddy.com'.

models.kt

/*
The idea behind it to make something like 'TravelerBuddy', but simplier. Easy to write and easy to read.
Just to have a chronological plan of points of interests and all crucial parts of journey. Also, to have all crucial info handy.

Backend is as simple as a CRUD (if you aren't planned to implement multi-user/authorization support).
The most work is expected on Frondend side. It needs to provide a convenient and very specific form for each TripItem.
And also draw a "graph" like below.

No need to support autocompletion for Address. It's enought to support "open in Map" feature. So even no needs in having a build-in map view.

Since there's no much commons between trip items (as well as no joins between them), 
it could make sense to use a NoSQL rather than have a ton of nullable columns in one RDS table.

10:00 -|- Leave the house
       | 
10:30 -|- Departure from bus station
       |
11:30 -|- Arrival to BER airport
       |
       |
13:30 -|- Flight departure
...
*/

data class TimezonedDateTime ( // very important to store timezone since trips could be between timezones
    val datetime: LocalDateTime,
    val localTz: TimeZone
) {
    fun toTz(val tz: TimeZone): LocalDateTime { // to show time according to the current timezone of the viewer
        // ...
    }
}

data class Trip( // simple container for trip items
    val title: String,
    val items: List<TripItem>
    val start: TimezonedDateTime,
    val end: TimezonedDateTime
)

data class DocumentLink(
    val name: String,
    val link: String // GoogleDrive Link, for example
)

data class TimelinePoint (
    val name: String,
    val time: TimezonedDatetime,
    val address: Address
)

interface TripItem { // abstract entity to represent different type of components of the trip
    val id: UUID,
    val note: String, // markdown - to support reference links
    val attachments: List<DocumentLink>,
    val timelinePoints: List<TimelinePoint>
} // all these fields should be represented in any inherited classes, ommited just for simplisity

data class Person ( // could as participant of the trip as well as any involved person.
    val name: String,
    val contact: String,
    val note: String
)

data class MapPoint(
    val longitude: String,
    val latitude: String
)

data class Address(
    val country: String,
    val city: String,
    val address: String,
    val mapPoint: MapPoint
) {
    val None: Address = Address("None", "None", "None", MapPoint("0", "0"))
}

enum class AirportCode {
    LEJ, AYT //, etc..
}

data class Airport(
    val code: AirportCode,
    val name: String,
    val address: Address
)

data class AirportPoint(
    val airport: Airport,
    val terminal: String,
    val gate: String,
    val time: TimezonedDateTime,
)

data class Flight: TripItem (
    val flightNumber: String,
    val carrier: String,
    val bookingCode: String,
    val seat: String,
    val passengers: List<Person>,
    val departure: AirportPoint,
    val arrival: AirportPoint
    // (?) connected (next) flight with type `Flight`
    // (?) return flight with type `Flight`
    // similar for LongLandTransfer
) {
    override val timelinePoints: List<TimelinePoint>
       get = listOf(
           TimelinePoint(
               "Flight from [${departure.airport.code}]",
               departure.time,
               departure.airport.address
           ),
           TimelinePoint(
               "Flight to [${arrival.airport.code}]",
               arrival.time,
               arrival.airport.address
           )
       )
}

data class Hotel: TripItem (
    val name: String,
    val address: Address,
    val reservationOn: Person, // the person who need to contact with hotel
    val guests: List<Person>,
    val numberOfRooms: Int,
    val contacts: String // phone; email
    val checkIn: TimezonedDateTime, // it's about planned time, not available from hotel
    val checkOut: TimezonedDateTime // for example, hotel offers check out from 11:00, but you want to leave at 8:00. So here should be set 8:00.
) {
    override val timelinePoints: List<TimelinePoint>
       get = listOf(
           TimelinePoint(
               "Check-In [${name}]",
               checkIn,
               address
           ),
           TimelinePoint(
               "Check-Out [${name}]",
               checkIn,
               address
           )
       )
}

data class PublicTransportConnections( // BUS 165 -> S 45 -> RE 5(3345)
    val time: TimezonedDateTime,       // [{ time: "2024-01-10 09:00", decriptions: "BUS 165" }, { time: "2024-01-10 10:00", decriptions: "S 45" }, { time: "2024-01-10 11:30", decriptions: "RE 5(3345)" }]
    val description: String,
    val point: Address = Address.None // no need to bother filling it every time
)

data class PublicTransport: TripItem ( // includes all potential changes, so use only one item even if you need to change from bus to train and then to tram.
    val startPoint: Address,
    val endPoint: Address,
    val departure: TimezonedDateTime, // could be approximate
    val arrival: TimezonedDateTime, // could be approximate
    val approximateDuration: String, // 1h 13 min
    val connections: PublicTransportConnections[], // optional
) {
    override val timelinePoints: List<TimelinePoint>
       get = listOf(
           TimelinePoint(
               "Start commute to '${endPoint.address}' [${approximateDuration}]",
               departure,
               startPoint
           ),
           *connections.map {
               TimelinePoint(
                   it.description,
                   it.time,
                   it.point
               )
           },
           TimelinePoint(
               "End commute to '${endPoint.address}'  [${approximateDuration}]",
               arrival,
               endPoint
           )
       )
}

enum class TransferType {
    Bus, Train, Ferry
}

data class LongLandTransfer: TripItem ( // it's about long transfers by land like train or intercity buses. Somethings where punctuality is crucial
    val transferType: TransferType,
    val transferNumber: String,
    val carrier: String,
    val contacts: String,
    val passengers: List<Person>
    val pickUpTime: TimezonedDateTime,
    val pickUpLocation: Address,
    val dropOffTime: TimezonedDateTime,
    val dropOffLocation: Address
) {
    override val timelinePoints: List<TimelinePoint>
       get = listOf(
           TimelinePoint(
               "[${carrier}] ${transferType} from ${pickUpLocation.address}",
               pickUpTime,
               pickUpLocation
           ),
           TimelinePoint(
               "[${carrier}] ${transferType} to ${dropOffLocation.address}",
               dropOffTime,
               dropOffLocation
           )
       )
}

data class GeneralPoint: TripItem ( // any point you want to mark on your journey map. For example, the very first and very last points of your trip.
    val name: String,
    val address: Address,
    val beHereAt: TimezonedDateTime
) {
    override val timelinePoints: List<TimelinePoint>
       get = listOf(
           TimelinePoint(
               "Be at ${address.address}",
               beHereAt,
               address
           )
       )
}

data class ObservationEvent: TripItem ( // not related to us, but important to observe, like friend's flight
    val name: String,
    val startTime: TimezonedDateTime,
    val endTime: TimezonedDateTime,
    val address: Address?
) {
    override val timelinePoints: List<TimelinePoint>
       get = listOf(
           TimelinePoint(
               "Start [${name}]",
               startTime,
               address
           ),
           TimelinePoint(
               "End [${name}]",
               endTime,
               address
           )
       )
}

data class Visiting: TripItem ( // visiting friends or places
    val description: String,
    val startTime: TimezonedDateTime,
    val endTime: TimezonedDateTime,
    val address: Address
    val persons: List<Person> // relevant if visiting someone, empty if no ther people involved
) {
    override val timelinePoints: List<TimelinePoint>
       get = listOf(
           TimelinePoint(
               "Start [${description}]",
               startTime,
               address
           ),
           TimelinePoint(
               "End [${description}]",
               endTime,
               address
           )
       )
}

Idea: add expense array to all TripItems with 4 fields: amount, currency, category (food/transportation, etc.) and note.

For sync, each item (and derivatives) should have 'syncData` property with following fields: createdDate, updatedDate, createdDeviceName, updatedDeviceName.

Keep an individual check-list per trip item. And then provide with one accumulated check-list.

For example, under the flight you can have "take passport" and "check hand luggage restrictions compliancy". But for a commute from airport to the hotel - "take discount voucher for airport rail express that I handed over from my friend".

And all 3 stuff user can see in one combined place as requirements before the trip starts

---
title: Antalya (Fethiye)
start:
  date: 2024-02-02
  timezone: UTC+1
end:
  date: 2024-02-13
  timezone: UTC+1
items:
  - id: d70e08f8-6504-4400-a0d4-15f338f8a043
    type: PublicTansport
    note: Commute from Home to the hotel in Leipzig
    attachments: []
    metadata:
      startPoint:
        country: Germany
        city: Berlin
        address: Kastanienallee 21, 12500 Berlin
        mapPoint:
          longitude: '72.4544810685576'
          latitude: '41.51634385867111'
      endPoint:
        country: Germany
        city: Leipzig
        address: Leipzig Hbf
        mapPoint:
          longitude: '92.4544810685576'
          latitude: '93.51634385867111'
      departure:
        date: 2024-02-02
        time: 19:45
        timezone: UTC+1
      arrival:
        date: 2024-02-02
        time: 22:43
        timezone: UTC+1
      approximateDuration: 1h 13 min
      commuteDescription: BUS 165 -> S 45 -> RE 5(3345)
  - id: ff6476ec-da38-4c74-9bb6-df33f2e382ff
    type: Hotel
    note: Hotel in Leipzig
    attachments:
      - name: Booking_1234.pdf
        link: https://drive.google.com/path/to/file.pdf
    metadata:
      hotelName: 
      address:
        country: Germany
        city: Leipzig
        address: Kastanstraße 42, 12211 Leipzig
        mapPoint:
          longitude: '62.4544456785576'
          latitude: '33.51634444867111'
      reservationOn:
        name: Katrin Mustermann
        contact: +49 177 1234567
        note: sample note
      guests:
        - name: Katrin Mustermann
          contact: +49 177 1234567
          note: sample note
        - name: Mark Mustermann
          contact: +49 177 2345678
      numberOfRooms: 1
      checkIn:
        date: 2024-02-02
        time: 22:50
        timezone: UTC+1
      checkOut:
        date: 2024-02-03
        time: 15:00
        timezone: UTC+1

Timeline design guildlines: https://experience.sap.com/fiori-design-android/timeline-view/

Instead of the month in the gray line, we can show timezone change warning

P.S.: Made in Lunacy