Just think about what you would want to control in the real world using software.
Recently I wanted to add bluetooth to my car stereo, so I got an arduino and got to coding.
maybe you would like a laser trip wire in your hallway that will set off an alarm in your room so your parents/roommates/strangers dont catch you "putting on the finishing touches"
you could also make an annoying throwie. wire up the microcontroller to make annoying sounds at random intervals and stick it somewhere stealthy.