S.B.
Not sure how many friends you're talking about, but what about a trip to the museum, aquarium, or planetarium? They may even have birthday packages if you inquire. Or go to the zoo and have a fun picnic. Of course you would need additional adults to help keep a eye on everyone too.
You could do something at home and pick a fun theme. Someone posted on here awhile back about a circus theme, and I've since seen companies where you can rent all kinds of cool stuff like popcorn machines, cotton candy machines, projector screens (for movies in the backyard!). I almost can't wait for my little one to grow up so we can do something like that. Have fun!