A huge number of man caves seem to fall into the sports-fan category. If you have any kind of sports memorabilia at all, it's probably rotting in a box somewhere if you don't have a man cave to display it in. Where else are you going to put it -- the living room? The master bedroom? We think not. The man cave is your chance to rescue your posters, jerseys and sports equipment from storage and put them where they belong, on prominent display. You can do a generic sports theme, of course, but a shrine to a specific team (or at least city) is really the best way to show your stuff.
Must-have: A multitude of signed items, and an original seat from your stadium of choice would truly be the piece de resistance.