Adventure Time is great fun (especially for fans of the show) but it's over in a few hours.
If you've played previous Layton games, you know Miracle Mask; more good stuff.
My favorites:
Ace Combat is surprisingly solid, especially if you enjoyed the PS2 entries.
Bit.Trip Saga is superb, but for me it requires headphones and complete concentration.
Cave Story (any version) is worth going through if you haven't.
I for one really champion Code of Princess, but I'm a big Guardian Heroes fan.
If you can get comfortable with the odd controls, Kid Icarus is as good as you've heard. I'm a southpaw, so it was a bit easier for me (for a change!)
Nano Assault is a lot of arcade shooty fun, kinda short but built for score runs. Cheap too.
The Pinball Hall of Fame is a somewhat uninspired port, but I love some of the included tables (such as Funhouse and Pinbot) and it stayed in my machine for awhile. Make sure to turn off the kinda cool 3D for a much more fluid framerate.
Resident Evil Revelations may be my favorite single player RE; great stuff for fans, more survival horror than straight action. Great atmosphere.
SMT: Devil Survivor may be an acquired RPG taste, but this version has more content than the under-rated DS version. I'm a fan.
Super Mario 3D Land is my favorite 3D Mario thus far, though the difficulty doesn't ramp up for some time.
Finally, the Ghost Recon game may not seem that interesting, but it's actually a turn-based strategy by one of the original X-Com creators. Not as good as the new Fire Emblem, mind you, but it is fun, cheap, and overlooked.
Some of my suggestions, anyway.