This is a simple, fun idea that I thought turned out cute.
Using contact paper cut out a heart. Then let your child decorate the contact paper. They could use a variety of different things from pom-poms to glitter. I thought it'd be fun to see what it would look like with torn pieces of paper. My boys picked out some paper to tear up and they had a great time putting the papers on their hearts.
This was simple enough that the only thing that I did was cut out the heart and peel of the backing. Then they went to town. (They are 5 and 3).