The problem is that there are too few decent 3D games for DS. A few include:
Crash of the Titans. Unfortunately, this game is probably not what you want. It's 3D, but linear like the PS1 games.
Tak and the Great Juju Challenge. This one is decent if you can get past the horrendous driving stages.
Transformers. There are 2 versions of this game- Autobots and Decepticons. I didn't really like the game, but some people did. It's a sandbox style game like GTA.
Rayman DS. This is a pretty mediocre port of the great Rayman 2. It's fully 3D though and used to be an excellent 3D platformer. Unfortunately, the DS port is plagued by choppy action, unresponsive controls, and blocky graphics. Still, if you've never played Rayman 2, and/or you can get over the flaws, it's still a good game.
Spyro Eternal Night is a fully 3D explorable game, but I really hated it. You MAY like it, but it's pretty bad.