Great question. We held a party at MIT during the Mystery Hunt two years ago; in the rooms that my team had reserved. It had quite a nice turnout even with only a moderate amount of cake and drinks.
We could try something similar over the long weekend. We weren't able to do the same last year because we were all running the Hunt and too preoccupied to think about such things; but there's a nice WP/Hunting cultural synergy.
We should definitely find a bigger room though - say space for 50-100 people to mill about.