Backstory: Yelp is the first app I think of using when I'm looking for a place to eat. The other day I was looking for a new local park and I realized I had to go to google and search multiple terms to find a small "greenspace" nearby.
The Idea: Yelp for parks and greenspaces.
Deep Dive: Very similar to yelp, you would allow people to upload and geolcate parks and greenspaces. Other would be able to rate and review the different spaces.
Validation: Yelp and Strava (bike locations) are proof that ratings for locations are valid products.
Monetization: You would have to sell ads targeted at outdoor activities or something similar.