When you open the card, three sides of the box fall down, revealing whatever you put in there. I have added balloons, a gift box, candles, hearts and a party hat. The back has a cupcake and a "happy birthday" talk bubble. There are also two "floating" hearts, which are secured to acetate.
I added a bow of turquoise and white polka dot ribbon.
This card was more time-consuming since I stamped, heat embossed and fussy cut each element. You could also use pre-made items.
Idea from Splitcoast Stampers.