Nintendo DS Jam Sessions
Not really a game, but Jam Sessions is great. If you ever played guitar or wanted to play then give this "game" a look in. It uses the DS touch screen to simulate a guitar string and holding down teh cursor pad changed the chord you are playing.
Within minutes you will have finished the demo and be playing a recognizable version of Yellow by Coldplay. Invest a bit more time into the demo songs and you will be playing others but the real beauty is in letting you experiment and coming up with your own riffs and tunes.
I love this as I can have a quick 5-10 mins of playing around between games. Look on you tube for some cool vids of people playing tunes on there DS to see what you can do with a bit of effort.
Trust Nintendo to come up with other ways of getting the good DS to entertain you.