Airberlin put up a new offer on its website for 1,000 free Topbonus miles merely registering with Sixt. Getting these miles don’t require any renting activity.
Sixt has also an offer Topbonus members that do rent from Sixt. They can get 1,000 miles per rental after 5 rentals, 1,500 miles per rental after 10 rentals, and 2,000 miles per rental after 20 rentals by May 31, 2015. The base number of Topbonus miles that you earn from Sixt rentals is 500.
You can access this offer on Sixt’s web page here.
Airberlin Topbonus has often ways to earn miles for nothing or very little partner activity. You just have to remember that the miles expire after 3 years of no use