1) Calculate object heights using the orientation sensor. Walk off (or use Google maps, or GPS) the distance to an object, then use the angle of base to top and trig to determine height.
–I checked The Play Store, and there are 2-4 apps that already do this.
2) Cute baby / ugly baby. Have people submit pics of babies, other users score them. A lowbrow app. Mostly used by friends or random passersby taking pictures of babies and submitting them. The app could also show the cutest / ugliest babies locally, by state, by country. Easy to implement. Definitely should be a pay app.