We all know Kryptonite came from Krypton. We'll assume that before the planet exploded, the stuff that would be Kryptonite was bad stuff. So what thing(s) that are here now would be Earthite when the pllanet explodes? Here's some possibilities:
George W's or Dick Cheney's skeleton (or whatever he has locked in that man sized safe)
Any form of any of the Baby Geniuses movies
Snapple (I know it's made from the best stuff on Earth, but what went wrong?)
(Insert that comic you really hate here)
Holiday foodstuff that comes around every year, but nobody eats it: fruitcake, giant jelly beans, candy corn, Aunt Glenda's special stuffing, etc.