You come across an unknown stamp and want some information about it.
An app that takes a picture of the stamp identifies it and return relevant information about it (history, value, etc.)
A mystery game between two friends.
You download the game app and invite someone else to join the game. All communication with them is done through our system, so they don’t know who you are. They get an invite explaining that you “the mystery friend” would like to play a game with them. The objective is for them to find out who you are. They can do that by joining the game. Every time you go somewhere, the app on your phone will ask you if you want to share that location with them (and when to do so – you might be with them at that moment and would like the info delayed.) If you do, they will receive a message with this new information. They have an X amount of time (you decide, maybe two days, maybe a week) when they receive information on your whereabouts, and until that deadline they can collect the information on you and try to figure out who you are. They can also ask for some clues from you, using the app messaging system. They have three tries to guess your identity. If they fail all 3 or the time runs out, your identity will be revealed.