Pool party, if one's available? Roller skating, picnic, group presentation about something (most of) you like, for example, cheese tasting, chocolate tasting, cupcake decorating. Go to one of those pottery places to paint and fire your own mugs or some similar shared crafty kind of thing.
Movie night and ask everyone to come dressed as their favorite movie character, have everyone guess who's who, and have small silly prizes.
Make it a "retro" night and gather a lot of the board games you used to play as a kid, find candies and things that were your favorites when you were little.
If you have a big back yard, how about a campfire with wood stick roasted hot dogs and, of course, s'mores?