Card Games
Easy to transport, card games are always a good way to spend time with grandchildren because they allow you to talk and spend time with them while keeping everyone entertained. You can choose a complex card game like Yu-Gi-Oh or Magic: The Gathering, or a simpler card game like War or Gin Rummy that uses a standard deck of cards. They're all good choices as long as everyone's interested in playing.
Board Games
Another old standby, board games, can involve a number of players, but they're best when you have at least three people. Whether it's a simple game like Candyland that's meant for younger children, or a more advanced game like Scrabble or Monopoly, anything that can keep your minds engaged while you are having fun is a good choice for a board game.
Puzzles
Puzzles are good activities to do together with the grandkids. Whether you're putting together a jigsaw puzzle or trying to solve crosswords together, puzzles allow you to work as a team to try and come up with the answers. If you want to make it competitive, you can get several copies of one puzzle and see who can solve it the fastest.
Video Games
Being a grandparent doesn't mean you're too old to learn something new. Many video games, especially new generation games on the Nintendo Wii, are meant to be enjoyed by the family together. This makes video games a great way to connect with your grandkids, and it might allow them to teach you something new, rather than the other way around.