Well considering OoT 3D and StarFox 64 3D are the only 3DS titles currently available that I was interested in, that's left me limited to DS titles and DSi software until 3DS games like Super Mario 3D Land, Mario Kart 7, and Kid Icarus come out later this year.
it sounds like you were in the same boat I was and skipped out on the DS and thus DS titles would work as well.
if that's the case, you are in luck as there are plenty of excellent DS and DSi games available.
You list LoZ Spirit Tracks, which is the newer Zelda DS title, but do not forget about LoZ Phantom Hourglass
Also, you might not want to bother with Mario Kart DS with Mario Kart 7 so close
Chrono Trigger for the DS is supposed to be the definitive version of the highly renowned game, and is only $20 new, so arguably a must have regardless of whether or not you've played it before
GTA Chinatown Wars is also pretty good (regularly rated as the best DS title) and can easily be found for $15 new.
If you like StarFox, you might want to check out StarFox Command (StarFox title specifically developed for the DS) which can be regularly found for less than $20 new (paid ~$15 for mine)
As far as downloadable games, I'd recommend LoZ Link's Awakening ($5.99 Game Boy Color Virtual Console), Cave Story ($9.99 DSiWare) and Plants vs. Zombies ($7.99 DSiWare)