In my senior year of college we had to make a restaurant pos system. Our group made a functional realtime android tablet app.

That allowed for customers to place order, request refills, play a very small game, chat with other tables, and pay for the check which supported splitting.

When an order was placed it strait to the cook part of the app. Which could view orders and complete them based on table number.

When an order was complete or a drink refill request the server part was notified. Where the server could view and finish orders based on table number.

There was also a very lite admin web component for basic reporting.

The UI was horrid, but we completed this in less than a month.