Münchers love the Isar River, which runs through Munich. It's a popular spot for walking, cycling, and in warm weather hanging out by the river's "beach"—don't be surprised to see naked people in some places. Müncheners aren't shy! There are also a couple of BBQ spots where locals bring out the grills and beer for an outing with friends.
You can start walking from almost anywhere along the river, but the Ludwigsbrücke (bridge), Maximilianbrücke or Prinzregentenstrasse are all good starting points, if you want to make a loop by walking down one side of the river, crossing the river, then taking in the view from the opposite side on the return way.