A friend of mine recently did a messy party for her dd's 4th b-day party. Everyone had a blast. She had poster paint with brushes (though these were not used for long) on the driveway (she ended up letting the kids paint her jeep and it is still that way a month later
), a pool with mud, a pool with water (though this did not take long to turn to mud as well), a sheet with a huge bag of flour and some bowl, sifters, etc... She had dough made up with bowls of sauce, cheese and toppings and the kids made their own pizzas for lunch and decorated their own cupcakes.
The only thing I would advise against would be the flour/water combo. My dd got her head wet and then dumped flour on top. She ended up with hair that was hard as a rock, kind of a paper mache helmet. It took a loooong time to get it all out.
It was such a great party, one of the best I have attended and I have been to tons of parties.