While looking for a valentine, I found this idea at Enchanted Learning. It was pretty simple to do. Use this idea for birthday, Get Well, or Thank You cards as well.
Option #2: Place a flat gift (sticker or small piece of candy) in the center. Fold all of the semi-circles in to create a card. Seal with a sticker. A larger sticker works better. Placing the folded cards under a heavy book will make the sticker seal better.