A few accessories that are simply "fun" and have no gameplay, or at least very minor gameplay alternation is the Wii-Wheel, and the Zapper. Both can be bought with a game (Link's Crossbow training, and Mario Kart wii).
Rechargable battery packs are very useful if you have the expendable cash.
Same goes with a awesome wii-robot that cleans your house an... wait. You mean they havn't invented a wii-robot? Dang. Back to the cyrotube I go.
I hope that helps!
Why must JRPG female leads suck so bad?