Also, have you thought about a video game tournament? You'd have to get x-boxes, PS2 equipment rented or borrowed and have kids sign up. I'd limit number of participants b/c some games run long. Offer snacks for those that lose quickly, and chairs so they can watch the others play. You could also do computer games instead of video games (especially if you have a computer lab)
Maybe the winner gets a GC for video games.
Other random thoughts: American Idol contest (use school name instead of American). Amazing Race. Dance contest (like the ones they have on TV). Think of the reality shows on TV and see if they can be adapted to your school.
There's a similar thread somewhere on here and someone else had a good idea--why not ask them?? Do a survey of the older kids and have them tell you what interests them. You can throw out a few ideas that may be feasible--like after school clubs on such topics as cooking, arts and crafts, board games--or evening activities like a movie night, ice/roller skating night--and then leave some blank space for them to put in suggestions.
We never have a problem with our K-4 grade students participating in any activities. And we never have a problem a thinking of activities for those grades.
There is a huge problem though with 5-8 grade students. I don't know what to do for them. I think they may be more open to the PTO and enjoy school more if we some sort of idea of what we could offer them.