New Super Mario Brothers is awesome. I also own Animal Crossing (dude, we should swap friend codes!). Elite Beat Agents is crazy fun. If you like rhythm games, I suggest you check it out. I bought Castlevania: Dawn of Sorrow for like $9 on clearance, another awesome game. (Reminds me of SotN kind of.) I have the new Zelda game and it's a must if you like the Zelda series.
I also own Super Mario 64 DS, but if you already own SM64 or didn't like it, it's not really any use to you. ^_^;
Mario Kart DS is really good, IMO. My little brother and niece both own it.
I'm looking into getting Brain Age 2. Bought Brain Age 1 for my little brother for like Christmas or something a long time ago. (Ideally, I'd love to get the DS Lite combo that comes with Brain Age 2.. since I'm still using the old, clunky gray DS.)
If I had to pick only 2 games to get: Mario Kart DS and Phantom Hourglass.