Poker is not just for the guys. The theme is friendly gambling- either for prizes or just for fun.
Setup a few card tables in the garage. Have music playing. As for the activity : the most popular game today is texas holdem poker. For the non-gambling type, have an alternative game or two. Also- have an instructional session for the newbies. Have a cheat sheet of what the poker hands so novice players can feel more confident during play. Give each guest poker chips, play money, and dice when they arrive.
Great decorations include hanging fuzzy dice and lots of black and red balloons.