In Reply To #1
Mario Kart DS
I mean, it's Mario Kart, non?
New Super Mario Bros
Well, it's a Mario jump'n'run and therefore a must have.
Final Fantasy Tactics A2
If you are a fan of both, RPGs and strategy and don't care about a free acessible world, this game can bring hundreds of hours of fun, if you really want to get all jobs and do all missions.
Mario and Luigi - Partners in Time
A really fun and nicely made Mario RPG. Everyone should play it. It's funny as hell at some times too.
If you liked the old Mana games, never EVER buy children of mana. It's basically a dungeon crawler with way-too-easy-bosses and like no diversion.
I dunno, there might be loads of other great games for DS, I just can't remember now.