Backstory: When visiting multiple cities and also for long trips, I'd love to have an idea of places I've visited a lot and new places to try.
The Idea: A heatmap of spots you've visited in a city based on how much time you've spent there.
Deep Dive: You would have to keep track of user's location and create a heatmap based off how long they've spent in different spots. You can pair it with local attractions to suggest other places to see if the users haven't been there yet.
Validation: I'd love to use this app now, I'm traveling with friends in Toronto and I feel we're wasting time trying to find new/different places.
Monetization: You would be able to sell targeted ads based of user's location and travel preferences.