I played SEGA's Crush 3D, the Nintendo 3DS remake of the PSP title, Crush, back at E3, and thought it was a clever little puzzler. With over 50 levels of mind-bending puzzles in four environments, it's the perfect title for the 3DS. It was probably cool on the PSP, but having the worlds in 3D really helps with the spatial puzzling.
Well, there is now a demo of Crush 3D available in the Nintendo eShop, and if you like puzzle games, I highly recommend checking it out. Especially if you enjoyed Pushmo. It's very different, but similar in some ways. And who doesn't like a good puzzle? Especially when you solve it!
And hey, if puzzle games aren't your thing, you could always test drive nintendogs + cats, Metal Gear Solid Snake Eater 3D or Mario & Sonic and the London 2012 Olympic Games! (Although assigning play limits to a demo is kind of annoying, I guess it's nice that we can download game demos in the first place...)