Summer in Munich is lovely, here you find some nice options for cooling down!
The Staffelsee is a great option for a day trip, or as they say in German “Ausflug”. You can walk through the beautiful village of Murnau and swim in the lake on the Murnau Riviera or Seehausen Riviera.
The Kochelsee is very easy to reach by train. From Kochel station there is a bus that takes you to Walchensee. Both lakes are ideal for a day trip or weekend excursion.