Our homeschool group organises our own science fair. It's for all ages (and the last couple years, it has become a hybrid science and history fair); the group rents a hall, sends the word out on the e-mail tree, the kids show up with projects, and everybody gets a participation certificate (there's no judging).

I don't know if that's the kind of thing in which you'd be interested, but it's simple to organise, lots of kids participate, and it's a nice celebration of the children's work.

peace
minnie