For card tricks, see Make a Wish from Caleb Wiles's High Spots. I also do a variation of the Anniversary Waltz where the spectator's selected and signed card changes into a card with a birthday greeting on the face (either using a blank card with a message or a regular card that has a message appear). You could also play around with any spelling tricks, having the spectator spell "Happy Birthday to find their selected card. Or my personal favorite, card to birthday cake.