M.M.
What about handprints in poster paint on paper, then putting them in inexpensive frames the kids have painted? You could even put the date on the frame.
If it's a longer party and you have another adult who can drive, you could take a pic of the birthday kid with each guest when they arrive, send the pictures up for one-hour printing at WalMart or CVS, then put the photos in frames the kids paint. They typically have basic wood frames at WalMart for about $2 -- or try the dollar store.
They also have cheap wooden boxes at Michael's or JoAnn Fabrics they could paint.