Dollar Store Crafter: Use A Dollar Store Plastic Pumpkin As A Mold For A Concrete Halloween Ornament - I feel like this would be fun to do with kids. Maybe you could even paint it when it's done? A permanent Halloween pumpkin, I love it!
Make your own tinfoil portraits/sculptures. You could buy face masks and give students tinfoil. I think this would work. Would probably look even better on a black canvas or cheaper option, black card.