Puzzle cards are an escape room experience in a birthday card. Some have your friends trapped in a room and using the clues that you seek out, piece together, and solve in the card - to try and escape. Others are more abstract sequences of puzzles that need to be mentally manipulated.
Just like an escape room, they can be solved alone or with friends. They are the perfect accompaniment to a birthday gift. The recipient must solve the puzzle inside to unlock a secret code. They must then tell you the code to receive the gift.