The Isar is Munich's river, which flows through the heart of the city. Its banks are splendid for relaxing. Although unfortunately there is no catering on the river banks, you can lie on the grass and enjoy the sun, play badminton or have a snack you brought along.
Tip: buy food for a picnic beforehand in a supermarket. Then travel to the Reichenbachbrücke bridge (Fraunhoferstraße underground station) and simply spread out on the banks of the Isar with your blanket.
Fraunhoferstraße underground station