A bit too sentimental for me, personally, but I like the pop up heart.
How to make the pop up heart
The pop up heart is a variation of the simple box.
Fold the paper you're using for your heart in half.
With dotted line of template on the fold of the paper, cut out heart along solid outside line. Cut along solid lines of smaller (pop up) heart.
With heart still folded in half, crease pop up heart one way, then the other.
Open up heart. Pop the small heart forward by pushing from the back.
Glue heart to the inside of your card, aligning the center fold of the heart with the center fold of the card.