Angry Birds is probably the most addictive game I have ever played. It's so much better on the iPad then on the iPod/Phone because the screen is bigger and it's in HD.
Plants vs Zombies is also really good for a simple tower defense game, and it's also in HD for the iPad
Other games and apps that I really like (not in any particular order):
Kindle E-Book App (for iPad - with the anti-glare screen cover)
Bejeweled 2
Chuzzle
Paper Toss
Tetris
Cut the Rope
As you can see, I spend most of my time playing games
There's some apps that I'd really like to be able to use, like Last FM or Hulu, but can't since I live in Canada and they're only for Americans.