Man cave art is completely relative to the man who occupies the cave. For some, it may mean a few favorite posters thrown up but for others the way the man cave is decorated is a reflection of the man himself. Nothing less than something cool or something that will be talked about long after everyone has left the cave will do. One thing is sure, decorating for the Cave must be Quick and Easy and of course fun. The easiest way to do it, is have someone else do it...but it still has to look good, the man cave has a reputation to uphold! One of the easiest ways to decorate those bare walls is to create a graffiti wall which can be created over time (memories in themselves) and everyone who enters the cave can help!
Framing the graffiti wall is optional, although sometimes it is a good idea to set boundaries of where things can be put before the gang starts drinking, just saying. You can even do multiple, smaller areas of "wall art", maybe even give one to the girls who you have to let in because they made the food. You know you don't want them messing up the "real art".
Now go ahead and start "tagging" the cave...be creative, let your imagination run wild, just remember who may be visiting the cave at some later date, the ladies, future man cave trainees. Your best bet is to keep it clean but still have fun. Maybe you can have a frame with a black curtain if you can't control yourself.