The final two DLC course packs for New Super Mario Bros. 2 will be arriving on North American 3DS systems on December 20th.  Each course pack costs $2.50 and will consist of three new levels.  The "Mystery Adventures Pack" centers around solving puzzles to find hidden coins and pathways, while the "Impossible Pack" pits normal, non-Super Mario against three levels devoid of any coins or power-up items.

Trailer (IGN/Youtube): http://www.youtube.com/watch?v=igOFOEA4q3k