Got the Zelda 3DS for last christmas and I've been lovin' it. Sadly it doesn't play my GBA games, but I have the old GBA SP or original DS for that. Here are some of the games I have for the 3DS that I liked:
Advance Wars: Dual Strike - Turn-based strategy game. Sorta reminded me of Warhammer 40K, the tabletop game, which I enjoy. Story might not be stellar for older folks, but I enjoyed it as a Kid and the gameplay is certainly top notch.
Lunar: Dragon Song - JRPG. I remember enjoying it, but I don't actually remember anything about it other than beating it and enjoying it :/ fortunately it was only 5$ when I was last at my local game shop, so it wont set you back any! Maybe I'll play it through again before the summer is over.
Castlevania: Dawn of Sorrows - Side scroller Action/RPG. I really enjoyed this one. Its a lot of fun, and there are tons of ways to customize your character. I really enjoyed the combat, too.
Golden Sun: Dark Dawn - JRPG. I really enjoyed the two GBA titles in this series, and while I haven't had a chance to play this one, game play looks to be the same, so it should be quite a lot of fun! Can't say anything on the story though :/
Pokemon Heartgold - JRPG. As an old fella, I really enjoyed the old pokemon games as a kid, so this one was right up my alley. Tons of nostalgia mixed with fun gameplay makes for a fun experience. Not the most awesome story, but I don't think you play pokemon games for the story!
Tales of the Abyss - Action JRPG. I really loved Tales of Symphonia for the Gamecube, and this is very similar. While I would prefer to play a ToS remake on the 3DS, This one is also a lot of fun to play!
The Legend of Zelda: Ocarina of Time - Action RPG. A great game. I played it all the time as a kid, and getting to play it again after all these years was awesome. Even if you haven't played it before, should still be a very fun game to play. I also heard a while back that they were looking at bringing Majora's Mask, it's sequel, to the 3DS, so I'm very excited about that!
Dr. Mario - Puzzlegame. You can get this off of the e Store for 5$ or so, and its alot of fun. Sorta like Tetris. Not as good as the version on the N64, but still a good game if you've got time to kill, and it doesn't take up your cartridge slot!
If you want, shoot me a PM and I'll be happy to give you a rundown of any of the above!