Steel Diver is pretty good, esp. the commander mode. Not $40 good but if you can get it used or for less it is pretty fun.
If you get desperate, I liked Rayman 3D but I rented it. Other than that let us know what you find.
I should have mentioned, I picked up Steel Diver too, but haven't spent much time with it yet.
i had a brief but intense addiction to the commander mode, it is quite cool trying to second guess the AI.. try it but dont blame me if you end up hooked.
Currently one of the downsides of the 3DS is the software library. Right now there is nothing but around the end of the year Super Mario 3D Land, Mario Kart 3D, and Luigis Mansion 2 come out.