If digital greeting cards just aren't cutting it for you, then you may want to try sending Greeting Games.
Greeting Games is a new type of greeting card from TinglyGames that lets you send a card with a game attached. The kicker is that the recipient cannot read the message on the card until the have beaten the game!
In addition to letting you keep your message a secret, Greeting Games also lets you add in-game messages and control the difficulty level of the game. The games are all modeled after popular games, such as Sudoku and "Fruit Ninja," so there's not much of a learning curve involved. Greeting Games if free to try until February 2014, when it will be moving to a subscription and pay-as-you-go service.