A robot to balance on exercise balls, and drive around.
I noticed there were an excess of about 3' diameter exercise balls around the office (people use them as chairs). I realized I could make a robot that balanced on top of them. It would be similar to a quadrotor in structure, except having wheels instead of rotors. And it could be a fun sensing and controls problem to solve (a two dimensional inverted pendulum). And everyone is making quadrotors these days. This is something similar in complexity, but different. And maybe version two can be a quadrotor. Or just add rotors, and have it hop from ball to ball.
Hardware: PJRC Teensy 3.0 Adafruit 10-DOF IMU Breakout - L3GD20 + LSM303 + BMP180