Theo Demo
See also Theo/Overview.
The online demo for Theo is at https://demo-theo.rattailproject.org/
Login with username "chuck" and password "admin" and you'll have semi-admin access.
Note that this Theo demo integrates with CORE-POS. If you need to integrate with a different POS then the menu would reflect your actual POS. There are certain other differences like that under the hood but the menu is the most obvious.
See also RattailDemo which describes a similar demo, meant to highlight even more possible features.