I would love to help you out as well...
*If the other kids are having "gym" parties, you could either have "gym" stuff to do at your house (hula hoops, basketball, t-ball, mini trampoline, ect) or go in a different direction as they might like the change. That opens you up to just about anything under the sun!
*Carnival - Can a friend (or another parent attending the party) paint faces (something simple -hearts, balloons, smiley faces)?? Lots of balloons. You could knock over plastic cups with a soft ball, set up a ring toss or try and get a ping pong ball (softer than a golf ball if you have to do it inside or someone gets hit with it) into a small bowl/cup.
*Pirates - You could do a treasure hunt around the yard/house, "dig" for burried treasure (pennies in sawdust, sand or packing peanuts), make treasure maps, walk the plank (balance beam, kind of tying it back to the gymnastics thing).
*Cars - Have races (you can get Matchbox/Dollar Store cars very cheap for each kid), roll "tires" (get inflatable pool tubes and have them roll those), make cars (wooden ones they could paint), if you have space/vehicles, they could draw a road on your driveway and ride bikes/scooters while some direct traffic, some ride.
* Generic Ideas - playdough, coloring, stickers, pinata, pin the "blank" on the "blank" (you can make anything and find something to pin on it), hide and seek, decorate cupcakes (to serve as the "birthday cake").
Good luck!
