you can make large 3 dimensional helium filled balloons with mylar that you cut and I think glue together somehow. I've seen them in someone's catalogue. Ice sculpture would be fun to try.
Sand sculpture....like the contests they have at the beach in the summer time. Clay, of course. You can call a foundry for sandstone and ask them to call you the first time they start over with clean sand in a casting series...if the first casting they do is an iron casting, it's pretty safe, if you use masks, for kids to carve on, and the love it. You can pour plaster in milk cartons and carve it or make molds with it. You can take tree limbs with branches and strip them of leaves and bark, then make abstract creatures, etc. with mixed media...paint them, add feathers, yarn, metal, etc. (See James Surls WONDERFUL work to see how he uses trees in his work.) You can use wood scraps and build some wonderful wall reliefs or table top sculptures with wood. You can create cardoard scupltures and papier mache them and add mixed media to them. You can sew and stuff fabric and make soft sculpture. You can take aluminum cans and cut them up and combine them with wire to make birds, planes, cars, and so on.
You can Use just wire and make wonderful sculptures that can be suspended in air, a la Calder, or wrapped with papier mache after making an armature of a funky person or animal and continue with mixed media embellishment. Look at Louise Nevelson and make "junk sculpture" that you spray paint, or make found object sculpture. You could tell the kids in the beginning that you want to collect all sorts of "stuff!" to use as resources, and start a junk box or pile that kids can use. You can use plaster guauze and do body casts. You can make masks, What age are you teaching? I think once you start thinking, you could go on and on.