Maybe you can get them an iPod filled with music your grandparents would enjoy. Pick their wedding song, the favorite album they drove you crazy with growing up, or hey, maybe your grandparents are hip and like Lady Gaga. I am sure either way this would be a fun gift from the grand kids!
Forget buying the card, make one! Grandparents are always proud to hang a handmade card on their fridge no matter what age their grand kids are. Get creative... what its not Van Gogh? or Hallmark? Believe me in your grandparents' eyes this is their grand-kids' "masterpiece"!
Think about useful things your grandparents will need this Fall. Perhaps some Fall accessories like a stylish new scarf, hat and gloves set.