Design choices
Features:
An user can create an account.
An user can log in with an existing account.
An user can start a travel within a zone
An user can end a travel within a zone
An user can see their old travels
An user can see their balance
An user can update their balance
An user can log out
Constrains
An user can not have more than 999kr on their account.
An user must have an unique username

SQLite:
My app make us of SQLite for it's database. The database consist of two tables:
AccountTable
TravelTable

The AccountTable has 4 columns: username:String, name:String, balance:double and password:String.
The TravelTable has 5 columns: start:int, end:int, date:Date, cost:double and username:String.
The two tables are connected by the user's username, which has to be unique. This is asserted when the a new user tries to create an account.
The start and end in the TravelTable is meant to be a zone, and I chose the zone to be represented by an int.