Backstory: Driving by baseball fields and volleyball courts, I've often seen them empty. I see an opportunity for a group of friends to play a pickup game for a smaller fee.
The Idea: Rent sports fields in off times at a discount.
Deep Dive: You would build an app where people could browse nearby available venues and rent them out. You could also offer asap referees.
Validation: This one can be easily tested before you spend much effort, you could concentrate on one specific sport at one location and advertise only for that spot, manually book and ref the first few games to get feedback.
Monetization: You would take a cut from the court fees.