This is a cheap and easy way to have fun and practice a lot of different skills.
Practice Sight Words: My kids LOVE working with their sight words when we use shaving cream. We have a "shaving cream day" for every 10 sight words we learn through our phonics program. I write the 10 words on the board, they write the words 3 or 4 times each and when they finish they can draw whatever they want. This usually takes about 20 minutes before the shaving cream disappears.
Draw Fractions: I had my kids using shaving cream to show fractions. They would draw the shape and then use their finger to divide the shape into whatever fraction I asked of them. You could describe a fraction and have them draw it. For example say: "Draw a fraction that shows 1 of 2 parts colored." They would draw a shape and wipe away 1/2 of the shape so that it would look like they "colored" the other half.
Draw a Shape: Describe a shape and as you describe the shape the students draw the shape. For example say: "Draw a shape that has 4 sides that are all the same length." The students would draw a square.
Math Facts: Either give the students the math fact and they write the answer or give the students a number and they can write what the equation could be. Use both for differentiation.
Vocabulary Words: Say the definition of a vocabulary word and the students write the word that you are describing.
This does clean up easily. I give them about a 1/2 cup or so of shaving cream and it usually lasts about 20 minutes. The more they play/work the faster it disappears.