I'm going to be driving from Salzburg to Fussen. I wanted to know some places that I could go to along the way to fill in a whole day.
Go to viamichellin.com and enter Salzburg as the departure point and Fussen as the destination. It is that easy. Try the various alternatives -- Michellin recommended, sightseeing, quickest, shortest, etc. -- and select the button for tourist destinations enroute. One convenent place is Chemsee (sp?) the island site for Ludwig's unfinished palace to rival Versailles. Or you can take the mountain route over the Alps. There several good routes and none is better than the other; each can take you past great palaces, churches. gorgeous lakes and other scenic places. Instead of basing your route on our opinions, when most of us have not tried all of them, experiment with different routes on viamichellin, and then make your own choice based on what you want to see and the time available.
Very good suggestions from Roy. Here's a route I like that has about 4.5 hours of driving time that gives you great views of the Alps, a visit to one of Germany's prettiest villages (Mittenwald), a castle (Linderhof), and a beautiful pilgrimage church (Wieskirche). Take autobahns A8/A93/A12 to just past Innsbruck and then go north on B177 to Mittenwald (good place for lunch). Continue to the B2/B23 toward Oberammergau and turn off to visit Linderhof Castle just before getting to Oberammergau. back to going NW on the B23 and then west on ST2059 toward Steingaden, stopping off to see the Wieskirche. From Steingaden, go southwest on the B17 past the royal castles & then Fussen.
The Ettal Monastery is also close to Oberammergau. Across the street from the monastery, there is a shop that sells beer that is produced by the monastery. The label has a picture of the pope. They also have a daily gregorian chant. Not sure of the time in the late afternoon--check their website. Have fun on your drive.
I don't know what time of year you are driving, but if the weather is nice and you are in Ettal or Linderhof, you might consider taking the drive along the Plansee to Rutte then Fussen. Lots of picnic areas and hiking trails on a lake surrounded by mountains. Take ST2060 from Ettal to Linderhof. After Linderhof continue going west, the road turns into L255 at the Austrian border then skirts the Plansee. The other posters are right in that there are many different great drives in the area. In additional to via Michelin, you can use google maps with street view to plan your journey. My last two points: if you are dropping your rental car off in Germany, it would probably cheaper to rent the car in Freilassing, Germany and avoid the drop off fee. It's a quick train 10 minute train ride from Salzburg. Also you should have a permit to drive a German rental car into Austria. The cost is around 8 euros and you can find them at gas stations along the border.