During the summer, people go tubing down Boulder Creek. To be fun, it has to be just right: enough water flowing, hot enough, and mostly sunny.
Objective: Build an application that lets the user know if it's a good day to go tubing. Implement it however you'd like using. Preferably Ruby on Rails for the backend/API, but beyond that whatever stack you feel is appropriate.
The goal is to demonstrate general competency, and comfort developing an open-ended application. Take about an hour. Since that's not much time, it's fine to describe unfinished parts with what you'd have liked to do.