Have the children make a list of what they are thankful for in their life. Then let them draw all of the things that they listed on their paper on a piece of white paper. Let them add all sorts of different shapes that they can cut out of multiple colors of construction paper and put them on with glue. when they are done you can laminate the placemat (you can have them help but keep an eye on them so they dont get burned) and either have them share what they are thankful for with the class or even with their families when their families come and pick them up.
I did this activity when I was in preschool and I absoulutly loved it, my mom still has the placemat that I made over 15 years ago :).