For 150 Coins each, you can choose Art Academy: First Semester for DSiWare (but only available on 3DS through this offer) or Mario Party 2 for Wii. If you get that one, use the money you saved by buying it with Club Nintendo money to buy some extra controllers, either for friends or to replace the ones ruined by Mario Party.
For fans of physical, tangible goods, there's also this new 3DS card case. It holds 18 games and includes four "double-sided card covers." And it looks really nice.