No, really. Cards from the Magic: The Gathering game. So called because, like crack, they are cheap but highly addictive and you never ever stop buying more, even when it's harming your job, your social life, and your health.
The term can be expanded to refer to any collectible card game, but Magic remains both the first and the best of these.
John bought another pack of cardboard crack, even though he just sold his left kidney to pay his rent. I think we need to do an intervention.
Cardboard crack refers to scratch-off lottery tickets. Especially to those who can't go to the gas station without buying a handfull of tickets and promptly redeem each two dollar winner so they can buy more cardboard crack.
My buddy is addicted, he can't go one day without scratching his cardboard crack.