Why is it some people like to pick on Neuschwanstein? Thanks to the French, there are not a lot of "authentic" castles in Germany. Some of the same people that bad-mouth Neuschwanstein recommend seeing the Hohenzollernburg in Hechingen or the Reichsburg in Cochem or staying in Auf Schönberg or Stahleck, none of which are any more authentic than Neuschwanstein.
There are buses from the Hauptbahnhof in Berchtesgaden to Schonau a. Königssee or to the bus startup at Obersalzberg (you can't drive up to the Eagles Nest; it's a private, bus only, road).
"The two big draws are Kehlstein (Eagle's Nest, Documentation Center and bunkers) and Lake Königssee. With a car, it's possible to do both in a day."
That's not true for me. I went up to Kehlsteinhaus, hiked around, had lunch, then came down and toured the Dok Center and bunkers. I spent almost the entire day. There was no time left for Königssee. Why even bother to go there if you are going to skimp on the time necessary to really see it.
As for side trips, in 2009 I went from Salzburg Hbf by bus to Mondsee (SOM wedding church), then to Wolfgangsee and Bad Ischl to Lahn (my overnight accommodations and the bus stop for Hallstatt were in Lahn, as is the parking for Hallstatt). What I paid for the bus tickets was about the same as for fuel alone for a car. The bus traveled on the same roads as you would by car, but because I didn't have to drive and watch the road, I was free to look at scenery.