Okay... back to the OP's original question. He didn't ask about gym memberships or running on trails, he asked about good Wii games.
I like Guitar Hero 3 and Metroid Prime: Corruption. The motion sensing controller and nunchuck in my opinion makes Metroid FAR more engaging than Halo or any other FPS on a console. Even opening certain doors adds to it when you have to reach out, turn a handle, and then pull the handle out. Very immersive!