Nintendo has a nice deal this holiday season for gamers in the United Kingdom. Register a 3DS or 3DS XL and a participating game between November 27 - January 13, and you'll receive a free copy of Super Mario 3D Land via eShop download.
Given the sheer quality of the applicable games, that shouldn't be too difficult to manage:
Super Mario 3D Land is a must-own for the system and, in my humble opinion, one of the first titles new 3DS owners should try out. That said, almost everything on this list is worth grabbing as well.