Popcorn doesn't seem like it is very easy to clean up in a lot of cases, but it can generate money. You just have to figure out which is more important to you.
During my high school's shows, we sell bottled water, pop, cookies/brownies, chips, etc. But in our last spring show we did the musical "**** Yankees", so we sold hotdogs. It actually wasn't very messy because the audience members ate the hotdogs before reentering the theater.
Great idea!!! (I mean, great Show-Specific Idea.)