I have found lots of useful things in my recycling and thought I might share them with you. I know boxes aren't very creative, but they are stackable and worth mentioning. But medicine bottles have lots of capabilities. They are air tight so they can hold paint without it drying, which I have been pleased to find out. The only disadvantage is that they are hard to decorate.
(My mom got a bunch of mini first aid kits for free with coupons, and she gave them to me.)