Quick n' Dirty Paint Cups, plus a Bonus Tip
One of the things my husband brought to our marriage is an insistence on good orange juice. I'd grown up on frozen concentrate and had no idea what I was missing. 8 years and 3 kids later, we go through a Lot of Tropicana Pure Premium. When they went to the new carafe style a year or so ago I started saving a lot of lids because they can't go in the recycle bin. Recently I discovered a perfect re-use opportunity: paint water reservoirs! With the wide base, they're virtually spill-proof as long as you fill only the central portion. (Needless to say, your kids will fill them all the way up to the top. They still won't tip, but they can slop, so you might want to keep charge of that task yourself!) They also hold just as much water as you need for the average toddler watercolor session - or even an adult ZIA watercoloring session.
And now for the bonus tip! I picked up a handful of aluminum cookie sheets at the Dollar Tree and we use them for Tons of projects. They're just a little larger than 8 1/2 x 11 paper, and they're great for containing (theoretically, anyway!) the mess from a variety of crafts. I also use them as a lap desk when coloring in front of the TV sometimes, or even to collect the worksheets my kids will be doing on a given homeschool day. Also, they're magnetic! I have a set of magnet-backed pattern tiles, and when I print out a template, even the two year old can build the picture right on her cookie sheet. Fun stuff!