Maybe too young for this yet, but my daughter said the most fun party she had was when she turned 10 at the Regal/Edwards theater in the Irvine/Tustin Marketplace. They have a couple of different packages, but basically, they rope off an area in the lobby for you to have cake and presents, then each kid gets a few game tokens to use. Each kid also gets a kids reel meal deal and you have a reserved row in the movie theater. The best part was the kids got to tour the projection booth and my daughter got to start the movie they were going to watch.
Also, Jump n Jammin in Mission Viejo is fun.