In my opinion, you can't go wrong with the following:

Ocarina of Time 3D

Professor Layton and the Mask of Miracle

Tom Clancy's Ghost Recon: Shadow Wars (it's a strategy game but it's a great game)

Cave Story 3D is a cool Metroidvania game.

since no one will ever dare mention it, i say Nintendogs plus cats, yes its not a manly game, but its pretty good if you have a heart.

but please...don't get these

Street Fighter , Dead Or Alive , Cave Story (get the Eshop version instead) , Steel Diver.

The only games I can recommend atm are Super Mario 3D Land (3D is awesome in that game) and Mario Kart 7.

Here's a link to some upcoming 3DS games is you're interested.

I recommend Resident Evil: The Mercenaries. It has a few drawbacks: only 8 characters, few stages, only two costumes per character, but in the end is a fun game especially if you play online. I have spent more than 60 hours and is still fun to me.

Cant go wrong with starfox.

That game was made for portables and 3D with all the lasers and what not.

resident evil revelations is coming out next year, kid icarus as well. so dont forget about those games.

You already had OoT, another game a could recomed Mario 3D Land, this game so simple and so addictive.

I recommed Samurai Warriors Chronicles, Dead or Alive Dimensions, and Pokemon Rumble Blast. Give these games an hour or two and you'll love them until you want to complete them. I also recommed you check out the e-Shop, games are starting to flow in there.

Are you trying to make the guy regret buying a 3DS  ? 

i actually like the Samurai/Dynasty Warriors series, but i don't recommend the Handheld ones

Dead Or Alive Dimensions is just lame, its trying too much to be graphical , but it fell short in content and is just really lame.

Pokemon Rumble Blast is no way a good retail game, even though i bought it on WiiWare i felt cheated, its very repetitive , simple and not really interesting at all.