Kids LOVE making a mess and getting their hands into something gooey. I still remember how fun it was to squeeze mud in my hands and feel it squish between my fingers as a child. This activity can simply be for fun or educational - depending on what you want to make out of it. We did both.
What you need:
A pan of some sort (I used a 9x13 casserole dish)
Shaving Cream (or if you have a child that likes to put things in their mouth, whip cream in a spray can)
Letter flashcards (optional)
I sprayed the shaving cream into the pan and smoothed it out with a spatula. Then I took out her letter flashcards (or you could write some letters on a piece of paper) and had her write them with her finger in the shaving cream. *I would suggest making a thin layer on the bottom of the pan to practice letters, we had a little too much shaving cream, which was great for playing but a little hard for writing letters.
Of course, I don't take credit for this idea, I've heard the idea from many different people/places - especially in the homeschool circle.