Michaels has kits where you can make a stepping stone. I did them with the kids for my parents for years, and they had quite the collection in their garden!
I also did a plaster handprint that could be hung on the wall - I'm sure Michaels still sells the kits.
Another one that I did for Fathers Day when both the kids were little was make a tshirt. I painted their hands with acrylic paint, then stuck it on the front of a tshirt with their names written underneath in puffy paint. I did the same on the back with their feet. It turned out very well, and DH still has the shirt!