Having watched the water cups around here multiply a few too many times, I finally decided that permanent water bottles would help maintain my sanity. We invested in Klean Kanteens, and because they were so pricey we tied ribbons around them and gave each child a bottle for Easter. They've lasted three years, partly because they are indestructible and partly because we don't allow them to leave our house!
Some of ours have been decorated with Sharpies and stickers, and now they come in some great colors so I bought a couple of those for the little guys. They match the kitchen ;)