my kids love glitter glue on a paper doiley. But that's a beast to clean up, and it's in the "fun to make" category, not the "turns out well" category.
You could take all the hearts out of an old deck of cards and attach them to construction paper with a message. I'd probably use 2 sided tape rather than glue.
I haven't tried this, but my daughter received a Valentine where you cut a circle out of felt and then close it (glue? tape? I don't remember) and it looks like a fortune cookie. So you put a V-day fortune on a piece of paper and put that in it before you close it.
Last year - pre-k was our first. We had a heart stencil which we used to trace and cut hearts out of construction paper. We bought googly eyes at the $ store and gave it eyes and drew a smile and hair if she wanted.
Let me make sure that my thoughts, words and actions are beneficial to the scene in which I find myself ~ Unknown
I like crayon resist water color paintings. I will draw with a white crayon on the paper and ds will paint with water colors over it. So for Valentines day we did hearts. He has now cut them all out is gluing them on other cards and adding stickers, but they would have been fine the way they were.