I'm not a grandparent, but the joys of teaching kids new things is immense. Like that if they put a slice of cheese in their parents DVD player they get a great video about cows, or of a slice of ham for one on pigs!
And gifts can be fun too; the drumkit I got my niece was a joy to watch her unwrap on Christmas day - the "grateful" look on her parents face brought a tear to my eye.
The main real joy though is the excuse to hit fun places like Science centres and watching cartoons guilt free!
Enjoy them while you can hand them back once they're sticky.