For a post like this, it would be nice to have a general idea of how old your son it. I can see, based on your history, that he's in elementary school.
Some ideas we've seen include:
paint a picture frame (you keep it, and return it with the thank you note and a picture from the party of that child)
Use permanent markers to "paint" plain white ceramic tiles (bought from any home improvement store.
Have kids decorate the bag that the favors will be brought home in (plain white gift bags available at any party or craft store).
Depending on how much time you have before the party, you might check with your local Home Depot or Lowes. See if they have any extra kids' workshop kits you can purchase (they're usually free for a single child, but they may not want to hand over 10 kits for free). They have all kinds of things like cars, pirate ships, tool caddies, and the usual bird house. Obviously, this would take more parental supervision (and would best be done outside). You might even ask them for aprons, too (again, usually given for free on Saturday mornings). Lowes has a pirate chest scheduled for this week and a helicopter scheduled for the 25th - either would be cool for the boys.
Boys could also paint balsa wood airplanes (the kind you shoot with a rubber band), but they're kind of flimsy, so they may not last very long.
Save some mayonnaise or peanut butter jars and have them decorate "bug jars."
I would also check out Oriental Traiding (.com)... they have lots of ideas and craft kits. Even if you don't get anything from them, it may inspire you with ideas.
Good luck! Sounds like fun!
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