I love my DS more than life itself, and am not seen anywhere without it. I don't care if thats sad

If you want good puzzle-y games, then you can't really go wrong with Brain Training and More Brain Training, and Sight training is quite fun too. Maths Training is tedious and horrible, but I'm crap at maths so that might be why I don't like it. Current favourite games are the Phoenix Wright series, a lawyer sim (I kid ye not, only japan could produce this stuff) which is amazingly fun. Kinda like the CSI games (one of which is out on the DS, good and puzzle-y), and if you like stuff like that try Hotel Dusk too. I also like pokemon, but that is because I'm sad and haven't grown up.
I'd recommend Super Mario 64 if you can get it, its a direct port of the 3d N64 game, but its awesomesauce and quite a long game to keep you occupied.
If you're a male and you like to shoot things your princess is in another castle, as Brothers in Arms and COD4 don't work so well with the touch screen interface.
Also, I'd say try and get an R4 card, to "view your pictures" or play "homebrew" games on the DS,
Gadget Asia are quite cheap for an R4 with a 2gig micro SD memory card.