How about the
Murgsee. You can drive up to the car park P1 and then there's a walking loop (probably 3-4 hours) with some beautiful views. It feels quite remote and desolate. At the See, there's a very
basic restaurant where you can have a drink and something to eat. You'll certainly be able to find somewhere to spend some time alone, either beside one of the lakes, or on the Murgseefurggel (or Wiedersteinerfurggel if you wanted to walk a bit further). The walk back down from the See is on a wider path and so would be OK in fading light.
Cheers
Jekyll