The concept is a simple one: a large container, cornmeal (polenta), and some odds and ends to put in the container. I think there are two options with the cornmeal quantity - go quite deep and hide items, or quite shallow and be able to "draw" in the cornmeal.
We went with the latter option, and I added craft sticks, a foam brush, plastic spoons and forks, and a cookie cutter. F immediately threw the cookie cutter out - that didn't suit him at all.
He then collected the other items, stirred the cornmeal around, and generally explored the new texture. I was worried that he would just tip the cornmeal out onto the floor but he didn't try once. I took the precaution of shutting him in to the kitchen (usually the other way round) just in case things started to get spilled.
Note - I've labeled this as "mess-free" because other than a quick brush down, F didn't need any cleaning afterwards. However, you'll probably want to have a vacuum cleaner handy for after your child has finished!