Backstory: Every Tuesday I play trivia at a bar with friends. Last week I left without my tab and had to walk back the next day to pick up my card.
The Idea: An app that allows you to pay your tab.
Deep Dive: You would have to sync with the bar's POS system. When a customer gives their card at a bar, the bartender would give the card back and the rest of the transactions would occur over the app.
The end user would be able to track their tab, split their tab with friends and pay their tab. This would save time and effort for the bartender.
Validation: There were nearly 15 other cards that were left behind at the bar when I went to get mine.
Monetization: You could sell ads for local bars and restaurants. Additionally you could take a small fee from customers for using the app to pay their bill.