Taps place a 30cm gold cube in the real world using your phone's own camera + motion tracking (no GPS, no server, nothing sent anywhere). Walk around it, get close, back away — it should stay put like a real object.
Occlusion test: after placing a cube, walk so a real wall, doorframe, or piece of furniture passes between you and it. If your device supports depth sensing, the cube should disappear when blocked and reappear when clear — for free, no account, no SDK.