I like the button/badge idea. if you get the button maker, maybe have the kids color on circles of paper the right size with colored pencils and add small leaves or wildflowers, maybe that they find themselves there if available.
Painting some sort of shaped wood, leather, or clay blanks might be fun, paint will take some time to dry but should be ok to touch in 10-20 minutes depending on the kind I'd think.
Or! Stringing chunky necklaces of wood or nut or clay beads. Natural, quick, portable, and fun. Available in bulk, your cost could probably be around $1 or so per necklace.