Low-Value Gift Card
Buy your friend a relative a gift card with a very low value of credit on it. Depending on how bad you want the gag to be, buy the card for a more expensive purchase, such as a meal out. The person will enjoy their meal, and then use their gift card to pay, which will only reduce the bill by a small amount, and he will be left to pay the rest himself. If you do not want the prank to be too costly, buy a coffee shop gift card, since this will leave the recipient with only a small additional amount to pay.
Fake Present
Wrapping a present up in a different box is a way of making a good present seem like a bad one, and vice versa. Again, how mean you want the prank to be will influence your course of action. With children, it is best to wrap a good present in an innocuous box, such as a games console in a box that says "Encyclopedias" on it. With an adult, however, you can reverse this and enjoy the look on her face.
Bad Present Game
The bad present game involves a group of people rewrapping the worst present that they received for Christmas. They all bring in their gifts and place them in a central pile. Each person takes a turn to pick out a gift, which he then gets to keep. This is a fun game for a group of colleagues to play immediately after the Christmas vacations, and is a good way of getting rid of bad presents.
Send the Gift Back
If someone gets you a gag present, or something deliberately tasteless, then giving it back as a gift is a game which can last indefinitely. Additionally, save the greeting card which accompanies the present, and reverse the names on the inside. Although this is not a wise tactic for someone you don't know too well, with a family member, it is usually acceptable and will be a game which lasts for years. If you have two siblings, make a triangle with the gift whereby sibling A gives it to sibling B, who gives it to sibling C, who then gives it back to sibling A.