An elaborate mass drinking device based on a sea creature, you can make up a horrible story surrounding how you caught it while swordfishing or something.
This one house that I have been to a lot has pages from porns all over the walls in one of the most used bathrooms.
You should make a pro-status pong stadium on one side of the basement, with ample room for spectators for when shit gets heavy.
This is an odd one, but if you are throwing crazy parties, it would be nice to set up some couches and pillows in a corner for the kids who are too fucked up to move. it might get puked on, but it would give kids a place to sober up before they fall down stairs or something.