We've done follow the leader before when you line them all up and basically walk around and around doing various noises/actions like train noises (with arms), clapping, stamping, funny walking, animal noises etc. They love this sort of thing but it requires making a right idiot of yourself LOL. Also simple games like statues (everyone freezes when the music is turned off) or musical bumps (sit down when the music is turned off) work well and you can make them either competitive or not competitive.

Prolly a bit late to organise now but other things we've done in the past is give kids strips of coloured cardboard to decorate (with pictures, stickers etc) and then staple them into crowns/party hats. We've also done temporary tattoos - the kids love getting them put on their faces .