This is an activity I did several months ago. It is easy to do, and great fun! Also, a great idea for the letter "F" - FEET!
We laid a long sheet of butcher paper out on the side walk. Then, we put several squirts of different colored washable tempra paint on it. We took of the children's shoes and let them walk (slide) around on it. They LOVED it!!
Remember that the paint is very slippery. We made sure there was a railing near by and walked closely to children to keep them from falling.
I had a tub of warm water and some towels at the end. The kids loved stepping and washing off their little feet.
The Foot Book, by Dr. Suess