Sand Castle Contest: while a sand castle sounds pretty dorky, pictures from previous years indicate that this is a pretty rad event.  If that isn't good enough for you, you can just stick around after and burn shit on the beach or spraypaint a dead whale.

This postage stamp of a photo was the only pic I could find of the event.  Also, I'm too lazy to look at Flickr.

Pacific Pinball Expo: I know everyone isn't as obsessed with pinball as I am, and we all know that having to travel to Marin for an event is torture, but there are 300 pinball machines in one room all weekend up in San Rafael.  Should be a good time.