We picked up these inexpensive foam letters from Michael's. How inexpensive you ask? Just $5 on clearance. All Mama's love clearance and while in my opinion Michael's is generally over priced, they have AMAZING clearance deals! My little guy enjoyed combining stamping as well as finger painting for this project. The only down side about these letters were that I had to cut them apart individually and did not think of doing it ahead of time. Plan in advance to avoid your little ones getting impatient while you cut apart the letters.
The foam was super easy to clean up and the letters are available in different fonts. I prefer these to wood stamps because you can combine techniques like finger painting or other types of paints (glitter paint, yogurt paint, coloring and so on) very toddler appropriate!