Is Berchtesgaden a reasonable drive out of Salzburg?
Yes, it is only about 30 min. But the main attraction in Berchtesgaden (apart from the Obersalzberg Museum) is once again the nature, people go to the national park south of the city to hike.
http://www.obersalzberg.de/
IMO it is not worth going to the Nationalpark Berchtesgaden if you do not plan on hiking there, plus at the end of October the weather up in the mountains can be too cold for hiking. You cannot enter the national park by car, you can drive to the border but then you need to take public transport or walk. There is a bus that goes from Hintersee to Weißenbach bei Lofer in Austria, this is the only option to see the Nationalpark without hiking.
See http://www.rvo-bus.de/site/rvo__bus/zubehoer__assets/de/dateianhaenge/bgd/liniennetzplan__bgd.pdf for the network of public transport around Berchtesgaden.
For a day trip out of Salzburg I would instead head to the Wolfgangsee in Austria, which is about 40 min by car in the other direction. Name of the village you should drive to is St. Gilgen. Take a boat, cruise the lake and take a ton of pictures!
http://www.wolfgangsee.at/en
http://www.salzkammergut.at/en/4-01-7-31931/list/salzkammergut.html
http://www.salzkammergut.at/en/4-01-7-32180/detail/salzkammergut.html
And for things to do in the evening in Salzburg:
http://www.marionetten.at/
[ Edit: Edited on Jan 15, 2009, at 1:56 PM by t_maia ]