Starting to teach them another language such as spanish or sign language would be good. Finger painting and talking about which colors combined makes another color. Nature walks and teaching them about the animals and the plants they see. Kids are natural learners and are curious about everything around them.
LOL, we do paper machet, which is safe and a fun mess. We also do finger painting in the bathtub using homemade edible finger paints lol. Now they are learning how to make peanut butter art...raisins with pb on crackers, putting things in cheap ketchup squirt bottles and letting them paint toast, crackers etc then eatthing the art.