a simple contract to let people check in and out and keep a count going

YourContract.sol

//SPDX-License-Identifier: MIT
pragma solidity >=0.8.0 <0.9.0;

// Useful for debugging. Remove when deploying to a live network.
//import "hardhat/console.sol";
// Use openzeppelin to inherit battle-tested implementations (ERC20, ERC721, etc)
// import "@openzeppelin/contracts/access/Ownable.sol";

/**
 * A smart contract that allows changing a state variable of the contract and tracking the changes
 * It also allows the owner to withdraw the Ether in the contract
 * @author BuidlGuidl
 */
contract YourContract {

    // State Variables
    address public owner;
    uint256 public checkedInNow;
    mapping (address => bool) hasCheckedIn;


    // Constructor: Called once on contract deployment
    // Check packages/hardhat/deploy/00_deploy_your_contract.ts
    constructor(address _owner) {
        owner = _owner;
    }

    // Modifier: used to define a set of rules that must be met before or after a function is executed
    // Check the withdraw() function
    modifier isOwner() {
        // msg.sender: predefined variable that represents address of the account that called the current function
        require(msg.sender == owner, "Not the Owner");
        _;
    }


    function setNewOwner(address someAddress) public isOwner {
        owner = someAddress;
    }

    function checkin () public payable {
        require(!hasCheckedIn[msg.sender], "Already checked in");
        require(msg.value == 0.001 ether, "Must pay 0.001 ether to check in");
        hasCheckedIn[msg.sender] = true;
        checkedInNow++;
    }

    function checkout() public {
        require(hasCheckedIn[msg.sender], "Not checked in");
        hasCheckedIn[msg.sender] = false;
        checkedInNow--;

        (bool success,) = msg.sender.call{value: 0.001 ether}("");
        require(success, "Failed to send Ether");
    }



    /**
     * Function that allows the owner to withdraw all the Ether in the contract
     * The function can only be called by the owner of the contract as defined by the isOwner modifier
     */
    function rug() isOwner public {
        (bool success,) = owner.call{value: address(this).balance}("");
        require(success, "Failed to send Ether");
    }

    /**
     * Function that allows the contract to receive ETH
     */
    receive() external payable {}
}