You can lead them to draw letters in the shaving cream if you'd like to go somewhere educational with this one, but I'm going to bet that if your kids haven't done this one yet that the first time they do, it will be hard to pull them away! C spelled WoWWW.
Clean up is easy- a wet towel. Some teachers use this in the schools because it cleans desks right up and smells good too.
*Variation: Let children run their toy cars through the shaving cream and check out the tracks they leave.
*Variation: The "no-mess" version is hair gel into a Ziploc baggie. Perhaps add glitter. Press to seal and also use clear packing tape to ensure that nothing is coming back out. Lay the bag on a flat surface and press on it to draw.