There are a lot of great, but underrated games on the Wii.
E.g. De Blob, a relaxing new Jump'N'Paint idea, with great leveldesign and fun gameplay.
Disaster, if you can handle the sometimes really bad textures and gfx, then it's a challenging and fun gameplay, with a lot twist, a hollywood like story with Nintendos (Monolith Soft = 1st party dev) first real great badass character.. hope there will be a sequel, but don't think so, because it wasn't such a big hit...

No more Heroes is the game with the best style in this whole fucking gen! The storyline with the assasins, the uber cool main character Travis with all his weaks and the art style with its 8bit icons and items! AWESOME!
Resident Evil: Umbrella Chronicles (if you are a RE Fan and want to have new background information) and House of the Dead Overkill for the fast and easy two player Rail Shooter fun, you can do nearly everytime and perfectly controllable with wiimote.
Little Kings Story: bought it yesterday and I LOVE IT!
It's so time consuming, so fun, so many nice details and great ideas... ♥ it!
AND ALL BIG Nintendo franchise like Zelda, all Marios (Galaxy, Strikers,...) , Metroid, Smash Brothers or Mario Kart, the new franchises like Excite Trucks or Excite Bots (never played it, wasn't released here yet, but I read the review), resurrected or polished ones like Punchout or the New Play Controls and even sometimes the Wii *** Games like Wii Fit or Wii Sport Resort (preordered yesterday) will bring you fun and longtime playtime.
I could tell you a lot more, but I think all of them will eta your time till next year, so have fun... (;