Midi Shoes
Here's a list of projects that people around the world are building using Arduino Nanos. Although it's unlikely that you will build one this advanced for your final project, it is possible and should give you some ideas of what you might want to make.
Haptic Headband - Wearable device that vibrates motors around a headband to indicate to where objects are to help blind people navigate.
Phil's Microtransat Boat - Autonomous sail boat attempting to be the first to cross the Atlantic Ocean from Newfoundland to Ireland. (See also George & Dragon, another boat in the competition using Arduino)
DIY Star Tracker - Camera mount and guide for astrophotography
Mars Rover
Ultra-sonic object alert for blind walkers
Weather station
Fire fighting robot
Clothing that responds to the environment