With paper mechanics, I try to come up with unique pop-ups that enhance the joke with a surprise. The design and movement of the paper mechanic should also support the subtext of the idea.
Here's an example. Some people, like myself, like to keep a low profile on their birthday. Others want to be the center of attention on their special day. I think the truth is that most people crave the outpouring of affection.
I believe this card has been a best seller because it works on two levels. For the reveler who loves attention, it delights the person who gets the card (the "recipient" as we call them in the greeting card biz). It puts them in the spotlight and works well in a group setting because everyone can read the writing on the back when the card is opened. In the case of the reluctant celebrant, the "sender" (the person giving the card) is pleased because it embarrasses the unwilling participant.
A high price greeting card has to have more than just a simple joke. It should say something about the relationship between the sender and recipient and have a funny payoff.