If you're looking for somewhere to while away a few hours for free (and soak up some history in the process), I highly recommend a trip to Tempelhofer Feld. The park is located on the grounds of the old Tempelhof airport, which closed in 2008.
Tempelhof began its life as a parade ground, and the original airport terminal was constructed in 1927 and has a colourful history. Tempelhof played a part in the dark fascist and wartime years when a production line was manned by forced labour. It also had an integral role during the post-war Berlin blockade and the refugee transport. During the Cold War, the airport was West Berlin's gateway to the rest of the world, and until reunification was controlled by the US Air Force.
Tempelhofer Feld has been open as a park since 2010 and is a great place to visit with friends for a BBQ, a picnic, or just to stretch your legs. It's a rather amazing experience walking on an old runway. It almost feels forbidden, as if passenger planes are still landing and departing on the grounds.
At the park, you can find runners, cyclists, people flying kites on their skateboards, and plenty of families taking the air. Tempelhofer Feld is also wheelchair accessible, and you can bring your bike, your roller-skates, or even hire cross-country skis for a few laps.
Venture a little further, and you'll find some hidden treasures: the old US Air Force shooting range, an old fire-fighter plane, and towards the terminal building there's a park constructed for the refugee families who have lived in the emergency shelters since 2015.