I'm a grown ass adult but I seriously LOVE Style Savvy: Trendsetters
If character customization in the sims/rpgs is your favorite, you will love it. Pretty much just a dress up game but there are a lot of other features & things to do too.There are over 10k clothing items in the game. I got mine for $12 at gohastings.
Elite Beat Agents for the DS is awesome if you haven't played it. Favorite portable rhythm game ever! It's under $10 on ebay.
I'm playing Super Mario 64 DS right now too. Still trying to get all the stars.
I haven't played it yet, but a lot of people are very into Fantasy Life right now. It's like RPG-lite with Animal Crossing style graphics!
If you want something closer to AC, you could do Harvest Moon. There are a few versions out on 3DS right now.
Phoenix Wright: Ace Attorney is fun too. It's more like a Choose Your Own Adventure book in game form, but it's awesome if you're into that. It's mostly text-based.