Use an existing game engine which supports native C/C++ code or develop your own small game engine

The engine must support OpenGL, textures, 3D models (only if it's a 3D engine), sounds and input

Use the engine to create a small game using the features outlined previously

Optional: model animation

Use the sensors and look for weird patterns of movement (the phone falls and then it stops moving, loud noises, sudden stops, etc.)

Ask the user for input

Send e-mails/SMSs containing GPS location if the user doesn't respond

Use the camera and GPS location to detect different features of the environment (QR codes, barcodes, faces, location)

Overlay pertinent information related to the detected features (for example, detect mountain trail markers and where you are on that mountain trail and overlay the map, or scan a painting and overlay wikipedia information)