What is the theme? Are you encouraged to do something calm, like a story, or a motor-skills activity?
When I have the option, I like "fun get the energy out" activities. The easiest I've found is a balloon game. Each kid gets a balloon and they have to see how long they can keep it in the air without touching the ground. I start simple - who can keep it up for 2 minutes? Now, who can keep it up for 1 minute using only their head? Now who can keep it up for 1 minutes using only their elbow?
Disclaimer - run it past the teacher and make sure it's ok. Also, it's best done on smaller groups of kids. If there are 25 kids, either split them into 3 groups or choose a different activity. Make sure there is a lot of room so the kids aren't on top of each other. And take the balloons already blown up, you don't want to waste the entire time blowing up the balloons and they never get to play.
I've done this 2 years in a row for classroom holiday parties, and the kids love it. If you have a theme - you can also use a few minutes of your time letting the kids decorate their balloon with a marker before the game.