123D Creature Show (free)
Autodesk's app lets you customise your photos by adding in fully poseable 3D characters. It's weirdly addictive, and will certainly liven up your Instagram feed if nothing else.
Sing! Karaoke (free)
Smule's hugely popular karaoke app has raised its game with its latest update, harnesseing the A7 processor to let you add real time effects to your voice, minimising your chances of sounding like an X-Factor reject. Probably.
OK, so this one isn't 64-bit, but rather is optimised for the iPhone 5s' M7 motion coprocessor to save battery instead (forgive us - it's still a good use of the hardware).
Moves is an activity tracker app that builds up a rich picture of your activity: exercise completed, time taken and locations visited that it calls your “Storyline”. It'll capture steps taken, miles run, minutes cycled and calories burned, and is even capable of working out when you’re on a car or bus. The visual representation it creates is rather gorgeous and super-simple to understand, and you can connect other apps to use the data you've collected, so your phone can become even better at making you fitter, healtheir, more productive.