I've shared that we have a sand table and a big tub of uncooked rice that we play in often. Well, the other day my daughter was doing one of those packaged art projects from Hobby Lobby, and she had a handful of sequins left over. I tossed them into the rice tub and let her dig for her buried treasure. She had a blast, and my mind started spinning with all of the things you could bury. You could definitely suit this to skill level burying larger items for smaller kids and smaller items for larger kids. If you're looking for a good 30-minutes of sensory fun, throw a handful of something into a tub of rice. Cheap and easy!
[Ideas: rhinestones, Matchbox cars, marker caps, bouncy balls, marbles, sequins, rocks, paper clips...]