iPhone hacks are pretty cool and this is one of the cooler of the cool ones. It’s an app that sends out an acoustic wave in order to measure the distance between two phones or between the phone and a microphone. While it’s not completely accurate, it’s still pretty fun.
To measure anything you need two iPhones or a wired mic. On one end, the iPhone or iPad emits a sound and on the other end there is a sensor to tell when the sound has been emitted. Presumably you could use your included headphones and I suspect a Bluetooth headset would inject some error, so you’re kind of constrained in terms of actual measurements.