Cardstock is a great one.
I covered a full page in paint and now its a piece of art itself.
Small glass bowls are nice too. I started using these the other day but they are nice for watercolors.
Ice cube trays are great if you are using lot of colors of mixing colors.
Plastic trays. (I LOVE these things. I have like 12 of them (plus I got them free). They are one of the best surfaces to work on for beading, painting, molding clay, or anything except baking (yeah... I accidentally tried that once and there were little brown circles under every cookie and a burning smell)