Need plan for day. This could be a long day of driving and stopping.
Spend some time in Berchtesgaden?
-- can we see any of the Eagle's Nest?
-- boat tour to the Church on the Lake?
Lots of sites along the route:
-- Schloss Linderhof
-- Garmisch/Partenkirchen and the Zugspitze
-- Oberramergau
-- Schloss Elmau (fancy resort now)
The Watzmann Therme in the town of Berchtesgaden offers a couple pools (salt and fresh water), massages, and other activities????
http://www.watzmanntherme.de/index_en.html
Berchtsgaden is also famous for it's salt mines. You can tour the Old Salt Mines, where you'll take a mine train through the mines and see how the miners extracted the salt. The air in there will definitely clear out your lungs!!
The Berchtesgaden Salt Works have been in operation since the 1500s, and they continue to mine salt today. The Salt Works are open from 9am to 5pm during the busy season (May 1 to Oct 15) and from 11:30am to 3pm during the low season (Oct 16 to April 30). The actual tour of the mine last about an hour, but you'll need extra time to put on your miner's clothes beforehand and purchase funny pictures afterwards!
Bobsledding??If you make the trip in winter, be sure to check the schedule and see if the bobsleds are running on the Olympic bobsled track right next to the Konigsee (The King's Lake).
http://www.koenigssee.com/berchtesgaden-bayern-urlaub/Article/ID/472/Session/2-yMBgc7uc-1-IP/Winter_Sports.htm
