You see those gift cards hanging on the rack near the checkout counter? Well, they're a total ripoff, and this activity will show your students once and for all why they should just never, ever buy them. And maybe you won't either!
Using a listing of 4 different values of gift cards from a popular "big box" store (whose name shall not be named here, except for the fact that they pay their employees substandard wages, prevent them from unionizing and then encourage them to go on food stamps....)