Sonic and Mario at the Olympic Games is a riot, but exhausting!
Ditto Donkey Kong (and dad and I remeber it from 1st time round!) but you can do this one sitting down!
We also enjoy the platform type games, like Super Smash Bros. Brawl.
But if you're only getting one, get one of the competitive type games like MarioKart, Olypmic Games or Donkey Kong. These are all about 6months - 1 year or more old by now, so you may get some bargains on them, or get used ones a bit cheaper. This is how we buy most of our games, particularly ones my DS9 wants like Pokemon or similar, you find the game geeks buy them, complete them in a week of two, then trade them back into the shops. Of course, because they've possibly only had them a short time, and because they are a bit fanatical about the games, they're always like new condition, and you get them usually about 20% off.