One of the most beautiful strolls you can take on a sunny day in Berlin is a walk along this liquid border of Kreuzberg and Neukölln. Start at the corner Ratiborstraße/Paul-Lincke-Ufer at the very Eastern end of Kreuzberg (or at Maybachufer/Neukölln) and walk all the way up until Carl-Herz-Ufer (close to metro Hallesches Tor or Prinzenstraße).
On your way along the waterside you will meet boule players, sunbathers and other followers of bohemian lifestyle - along with plenty of nice cafés and beer gardens to quench your thirst.
Even though it may be tempting at some spots, taking a bath is not allowed and wouldn´t be much fun either - the waters are too polluted.