Most everything people see in Munich centers around Marienplatz, which is a short S-Bahn trip (2 stops) from the Hbf. There you will find the Neues Rathaus (city hall) with the Glockenspiel (playing clock, 11 am), Frauenkirche (the church with the green cupolas which is the symbol of Munich), the Residenz, Viktualienmarkt.
From the north side of the Hbf, you can take the streetcar (#17) out to Schloss Nymphenburg, the summer palace of the Wittelsbachers, with it's extensive park and outbuildings.
You can also take the S-Bahn from the Hbf to Dachau station, where you can catch the bus (well marked) to the memorial, but I don't know if I would call that "relaxing".
When you get to Munich, purchase a "Innenraum Erwachsene Partner Tageskarte (all day transit pass for up to 5 people) for €9,40. That will give you unlimited rides all day on all of the conveyances (U-/S-Bahn, streetcars, and buses) in the inner zone of Munich, which includes everything but Dachau. To include Dachau, get a Munich XXL Erwachsene Partner Tageskarte for €12,30. That will include the S-Bahn (S2) to Dachau station and the RT bus to the memorial.