Hi -
There are many different directions you could go. Here's one great site you might find interesting:
http://sites.google.com/site/pinczakko/
Frankly, you might find a LOT of interesting stuff by Googling for the word "hack", e.g. "firmware hack" =>
http://www.popsci.com/diy/article/20...-hack-firmware
http://www.maximumpc.com/article/fea...s_new_software
http://www.networkworld.com/community/node/41672
<= These are just the first three hits I found...
... and any one of them might be a cool project that's both fun
... as well as a great way to get your hands dirty with firmware programming!
Happy hunting ... and have fun!