There are two bonus stages to unlock in Mario Party 9 for the Wii.
Here are the bonus stages, and how to unlock them.
Bowser Station – Complete story mode, or play every board except DK Jungle Ruins in party mode
DK Jungle Ruins – Purchase for 500 party points at the Museum
Check out my other article for the complete list of unlockables in Mario Party 9 for the Wii.