How about a backyard party with lots of silly races and games, hide some candy (like lollypops or something small) around the house and do a hunt, maybe have little prizes for the winners of each activity?
Or go to a pizza parlor and just have a lunch party, maybe hand out some pen and paper activities the kids can do at their tables (word searches, etc).
Chuck E. Cheese, bouncy houses, pool parties, bowling parties, McDonald's/pizza parties, backyard parties, etc. are all popular in the US.