We went to the store the other day, and found a couple of $2 waterbottles, made by Rubbermaid. After we washed them out really good, I put some newspaper on the table and got down my rainbow collection of Sharpies (which, I admit, I am very selfish about - they just don't ever get to use those - but, hey, they're permanent!), and told them to have fun decorating their new bottle.
The cool thing is, the color will rub off eventually, and they can do it again later. Easy and fun, and they love these things!