Kids that age love to be creative and goofy...emphasis on the goofy. What they have fun with doesn't need to be functional or to our liking ;-)
I would either do the PVC bows or arrows (again) and have a "best dressed" contest...
They dress up their item of choice (or your choice) by finding items via a scavenger hunt. The stuff they find is either glued or tied to the arrow or bow...leaves, grass, pine cones, feathers, nuts, rocks, gum wrappers, paper, etc.
Sound crazy??? How do you think the first snake skin got glued to a bow? No adult would think of that :-)
After the artwork is done, judge the items by different categories...e.g. most natural material, most cleaver item used, most unusual item used etc. but don't judge on beauty...that is in the eye-of-the-beholder and the young artist.
Maybe use some of your money to hand out some small prizes...or maybe the vendors will have small trinkets to give...
I would think the goal is trying to give them something fun to do - for a given period of time. The item created isn't likely that important part of the effort nor is the prize.
my 2 cents....