Drift and Beanie and the Balloonist are two free apps that get you up and moving. Both have you follow a path along streets in your neighborhood to accomplish a goal, but with very different interfaces.
In the game-like Beanie and the Balloonist you are walking the streets to "collect objects" on your iphone or ipad in order to win the game.
Drift is more of a scavenger hunt that has you walking around the neighborhood collecting things through photographs. You'll get directions such as, "Walk north for a block, find something old beside something new and take a picture of it." Learn more here. FYI: your pictures may get posted live of the Broken City Lab website under the Recent Drifts banner.
Both are fun, and whether or not you actually play these two apps with your students, you can learn something from their design. The next step is to mimic either game to create your own environment-driven tech challenge.