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.
Start with the template. Cut it out on construction paper. Write whatever you want on the inside. Sweet Pea signed her name at the bottom.
Option #1: Fold up the bottom two semi-circles to create a heart. Place a sticker to hold the bottom two up. Tape a candy or sticker in the bottom.
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.