I like to use zero-balance cards for household tasks, so I keep a bunch around.
An example would be drywall repair. Old cards work just as well as joint knives to spread and flatten spackle (joint compound) when filling small holes.
Another example: I recently had to repair some spots on a wrought iron fence that had rusted through. Bondo (or whatever product you're using to fill holes) needs smoothing after you apply it.
Another example: old cards work well with any wood filler, so I use them on wood projects as well.
I do confess that I then toss them in the trash after they've been reused on some home project. But at least the cards have had a second life before they are discarded.