If you have some gamers , lan parties during work hours is always fun, maybe combining it with the beer fridays. We would bring in Xbox's sometimes as well and play multiplayer on there. Plus its a team building exercise.

Do it once in awhile during office hours helps a lot. As much as I love my co workers, somedays I hated spending time with them all day, and then sort of feeling forced to hang out with them after work or else feel like not being part of the team.