Do you have Pump It Up there? It's an inflatable warehouse space...if not you probably have something similar. Anyway I know that at PIU, when you book a party the party just automatically includes 25 kids (not sure about adults, but I think it was pretty open) regardless of how many you invite. This might be a good option b/c it will include everyone's friends and I think it costs about the same as a "normal" party about anywhere. They do basically everything for you, even cut the cake and write down the presents!
Another option might be a skating rink. I like the movie idea but wow is that a lot of M&Ms and Twizzlers! ;) Plus it would be hard to keep track of that many kids in the dark.