On the non art side of game making, creating enemy AI is my favorite thing to work on.  It's really fun to bring a character to life but it's also really difficult for me to code.  I get lost in that pretty easily.  I just finished up the AI for an enemy which I'm calling the Exploder for now.  Here is a preview video showing off what he can do.  This one has the most basic AI of the enemies I'm planning but it's still pretty fun to play around with.  The graphics for him are still work in progress but I'll probably be keeping the basic design.