Here's a really simply sign you can make. I just used poster board that I had and paint that we use for finger-painting.
I used foam letters that you can find pretty cheap at Walmart or the Dollar Store. Then we simply stamped the letters with paint to say "Happy Thanksgiving." My daughter was really patient as she helped me stamp the sign, so I let her put one finger in the paint and put leaves on the sign. After we finished this craft, I had some extra paper ready so she could paint at will. I always have something extra in case she's not done when I am. She also colored a turkey that we glued to the sign.
This could be easily done for other signs or other projects.