Despite the kiddy look, Chocobo's Dungeon is a roguelike RPG with quite a bit of depth. Many jobs (or classes) to use as the dungeon contexts change; dozens of items to discover, customize, and equip. The JRPG translations and some of the voice acting can be a little grating, but the bottom line is this is a fun game with several dozen hours of content.
If you're looking for a solid turn-based RPG on Wii with a respectable level of difficulty, Chocobo's Dungeon is a good deal.