When my son turned 3, I blew up maybe 15 balloons and put a piece of paper inside (index card size) rolled up small, with a number on it. The kids had to sit on/pop the balloons. The person who found the number "3" (because the bday boy was turning 3yrs) was the winner. In hindsight I wish I'd put two or three 3s in balloons instead of just one 3. That way there was a first place, second place, etc. But all kids could participate and all kids in attendance knew their numbers 0-9, so it was very easy.
My son just turned 4 and he didn't want to play this year, but I did consider having pictures of superheroes in each balloon (instead of numbers) and having kids find one particular superhero or maybe making a board where they had to work together to pop the balloons, get the superhero picture, and then match it up with the superhero's name. I guess if I'd have done that it wouldn't have been quite a 'game' where there's a winner, but an activity none-the-less!