Open-data version: real 360° photos from Panoramax, a community street-level photo map, and an OpenStreetMap guess map. No account, no key. 5 rounds, 5000 points each.
Challenge a friend
They replay the same 5 photos and see how they compare. Nothing is stored anywhere: the link carries it all.
Play together
Everyone gets the same photo, guesses on their own map, and sees all the pins together at the end. Gentle, no rush.