Personally, I'm partial to lighting. It's easy as heck to rig up, too. The cheapest and easiest lighting mods (
IMO) are:
A nice bright cold cathode kit, Some bright EL Wire (it's easily attached with epoxy or super glue)
and
fans with LEDs on them..
Make sure you look around a little bit and find the best prices. Also, there are pictures of modded cases all over the internet. Check some out before you screw with your case for best results. Good luck!