Backstory: Recently I went on trip with Julia to Toronto. We took a local bus tour where you hop on/off at stops. There was number to call to see where each bus was located but they just looked up the GPS of each bus and told us a rough approximation when to expect the next bus.
The Idea: An app for local bus tours.
Deep Dive: You would build an app that would tap into the GPS location that buses already transmit. It wouldn't have to be fancy and it would give an exact location of each for local users.
The value to the bus tours is that you would eliminate the need to pay for someone to maintain the phones to tell callers where the buses are located. You would also give a more accurate/real time location of each bus.
Validation: I'd definitely download this app, we ended up waiting 10 minutes past the expected time given by our phone operator.
Monetization: You should try to get a city to pay for a beta version but you would own the IP. You could then offer this service to other cities and have advertisements.