When my oldest daughter turned 4 we did a costume party. Her birthday is in June and it was perfect. The kids loved wearing their costumes in the summer. I set up stations around the yard and they just went to whatever stations they wanted. The stations were
*Be a Chef - had a chocolate fondue set up with cut up fruits, pretzels, marshmallows, etc. to dip in using toothpicks
*Be a rock star - music and microphones
*Be a fisherman - a wading pool with magnetic fishing poles and magnetic fish
*Be a fireman - squirt bottles and red cups decorated like burning houses (used craft foam)
*Be a Mad Scientist - eye droppers, container of vinegar and little cups with baking soda in them, they put eye droppers of vinegar into the baking soda and watched the reaction, this was a favorite
*Be a baseball star - balls and a target
*Be a pirate - sand with pennies and fake jewels in it and shovels to dig with
*Be an Artist - an art project
It was a blast and honestly the best party we ever had