I've always tried to devote as much time to my work as possible, whenever possible. However my job drained the energy out of me and made any effort I put in become spread out over multiple little projects. The main reason I'm asking this here is because I have a nice amount of time to sit down and work on one little project until completion.
Honestly if I can hold it in my hands and it works in a unconventional way, it's brilliant.
Another poster mentioned GB development, I honestly think controlling external devices (AVR, Arduino, etc) in a standard way using something like a GameBoy would be a worthwhile project.