Zumba Fitness World Party

Bonus content is unlocked by obtaining at least three stars on a song. Each song you complete with at least three stars will reward you with postcards, souvenirs, etc. In order to unlock this achievement, you'll need to complete every song in the game with a rating of at least three stars.

In general, this is incredibly easy to do. The songs were designed to be easy to learn so that you can dance along to lose weight and get a workout. None of the moves are complex, and only a few moves involve some intense movements. The majority of the skill needed for this relies on having the energy to keep on pace throughout the songs. Each is pretty short though, so if you break up your sessions you should have no problem obtaining a three star rating on every song, unlocking this achievement.