The standard fiscal POS app was adapted to support a sort of low-trust swarm of waiters who used the app to collect orders. These orders were then transferred to a few high-trust cashiers by scanning QR codes generated on the waiters' apps.
After receiving payments, the cashiers' apps printed invoices and multiple "order tickets" categorized by "food," "drinks", "sweets"... This allowed waiters to retrieve items and deliver them to customers.
The system was used by around 40 users, with new waiters joining or leaving throughout the event. They used their own phones, and the app functioned without internet or Wi-Fi, gracefully downgraded (If a waiter didn't use the app due by choice or due to technical problems, they could manually relay orders to cashiers), Customers also had the option to approach cashiers directly, receive their order tickets, and pick up items themselves.
This is not that technically interesting, but I liked how the old manual system, the 70+ year village firefighting org. main cashier had, got digitalized in non-centralized way. (and I took this chance in trying to explain it, as I will have to, to maybe find more users for it)