All three universities offer a big variety of sports courses (Sailing, Paddling, Boxing, Kung Fu, Yoga, Pilates, Salsa, Rock'n'Roll, Badminton, Volleyball, Tennis, Climbing, Theater, just to name a few) at a low price. Just check out the university sports websites below:
Many sports courses are quite popular and quickly booked out so it is best to book directly at the time when the booking is opened. The booking is normally opened at the start of each semester (the date is announced in advance on the website). So if you want to be sure to get a certain course then check the sports website regularly on that date (at least once each hour) and book as soon as the booking is opened. You will need your immatriculation number and banking details for the booking.
Alternatively, Berlin also has a big variety of sports club ("Sportvereine") which offer all kinds of sports. Here you will have the chance to do more competitions and meet not only students, but people from all ages and backgrounds. Usually, you will have to become member of the sports club.