We really love air-dry clay. We made these hearts using small cookie cutters, let them dry, and then painted them and glued magnets on the back. They can break if you handle them too soon. I usually help them dry faster by putting them in a low oven for a few hours during the day. The letters are from this great cookie set (here's the set, but I can't find the kit online anymore, sadly.)
I found these great printable sheets for simple science projects/experiements you can do at home over at bill nye's website.