2nd time in Munich

The trip by train and bus to the Eagles Nest takes over 3, maybe 4 hours, so 7 or 8 hours RT. Even if you spend 4 hours at the top, that's longer to get there than to be there. There are other things to see in Berchtesgaden, but even if you spend get up early and spend eight hours there, it's going to be a long day.

Dachau, on the other hand is easy in a day. It's a suburb of Munich and only about 40 minutes by S-Bahn and bus from the Hbf S-Bahn station.

OTOH, it takes only about an hour and a half to get down to the Alpine village of Oberstdorf and the nearby Austrian valley of Kleinwalsertal. Or you can go a little farther to Lindau on the Bodensee (Lake Constance) or to Friedrichshafen and the Zeppelin museum.

If the weather is nice heading to one of the lakes south of the city is not the worst idea in the world. Ammersee, Starnberger See and Tegernsee are all easily reached by S-Bahn or train.

A visit to a Kloster Brauerie is highly recommended. You will find a lot of info on Andechs in the forums but I always like Kloster Reutberg (easily reached by taxi or bus #9553 from Holzkirchen S-Bahn station). http://www.klosterbraeustueberl-reutberg.de/

DJ
(edit to include bus reference from Lee)

"Kloster Reutberg (easily reached by taxi from Holzkirchen S-Bahn station)."

There us a bus, #9553, from Hozkirchen Bhf, 17 minutes in the direction of Bad Tölz, stopping at Kloster Reutberg.

The lake district south of Munich (Kochelsee, Teggernsee, Schliersee) is definitely "off the beaten path". You can get there by taking the BOB train south from the Munich Hbf or the S3 S-Bahn to Hozkirchen. The town of Bayrischzell is probably the most "off the beaten path".

Here's my favorite easy day trip from Munich.

Take the S8 train southwest from Munich to the end of the line at Herrsching. Walk 500 meters to the pier on the lakefront. Take one of the elegant paddle-wheel ships across the scenic Ammersee to the authentic, untouristed lakeside town of Dießen am Ammersee. Wallk through Dießen to the ornate Marienmünster abbey (1732) that overlooks the town.

Return to Herrsching, and take Bus 951 (or hike about three miles up into the forested hills) to Kloster Andechs, where the Benedictine monks have been brewing beer since the 15th Century.

Photos here.

Or take S1 about 20 minutes from Hauptbahnhof to Oberschleißheim, and visit the magnificent, colorful Schleißheim Palaces, summer home of the Bavarian royal family. Aviation history buffs will enjoy Flugwerft Schleißheim (aviation branch of the Deutsches Museum), a short walk from the Schleißheim Palaces.

If you have a free Sunday, go to the spa town of Bad Wörishofen (about an hour by train west of Munich), birthplace of naturopathy. Therme Bad Wörishofen is a wonderful spa/sauna/waterpark complex. Then go to the open-air cafe on the small grass-runway airfield on the north side of town and watch skydivers do their thing, while you wait for your 45-minute ride in a classic 11-seat, Russian-built Antonov An-2 biplane (advance reservation required; photos here).

Tourists have yet to discover Ingolstadt, a charming, friendly town about halfway between Munich and Nürnberg. There are many historic buildings in the old center, including the former Anatomy Building of the University of Ingolstadt, now the German Museum of Medical History. Gardens in the courtyard are medicinal herbs and plants. The building was the setting for Mary Shelley's original 1818 novel Frankenstein. Tours are available of the Audi factory, just outside the old center. Ingolstadt self-guided walking tour available here.

In Munich itself, walk west along Briennerstraße from Odeonsplatz, past the small park dedicated to victims of the Nazis (Platz der Opfer des Nationalsozialismus), to the new NS Doku-Zentrum, scheduled to open in April. It is built on the site of the wartime Nazi party headquarters, destroyed at the end of the war. Just around the corner is the three-story School for Music and Theater. During the Nazi era this building, then called "Führerbau", housed Hitler's offices. In a second-floor office (now a practice room for music students) Hitler, Mussolini and Britain's Neville Chamberlain signed the infamous 1938 Munich Pact. Across the street is the historic Königsplatz.

Video of many of the above sights, from August 2014, here.

I recommend the BMW Factory tour (reserve ahead), the Flugwerft Schleisheimm (above), and the Munich City Museum (for both permanent and special exhibits). Nuremberg/Nuernberg is also day-trippable, only 1 - 1.5 hours by direct train.

Another vote for Dachau and BMW Museum. Dachau is frequently full of teenagers on school trips, so it is very appropriate for that age group. I'd leave that as the only activity for the day you go, as it's emotionally draining. If you can, take one of the free walking tours that meets in Marianplatz every day. While you know where you're going, it's kind of fun to hear background stories of the places you've seen. I liked Sandemann's a lot-they also do an excellent guided tour of Dachau.

The former site of the Olympics is unique in that it's one of the few that has been successfully converted to proper use by the city. The aquatics center has been converted to a public pool and there is a hill comprised of rubble from the war you can climb for a fantastic view of the city and the mountains in the distance. It's also a good place to explain why both Germany's Summer Olympics were failures and why they're pretty reluctant to hold any since.

Allianz Arena (home of Bayern Munich) also conducts daily tours in English. I've never been there, but it looks like a beautiful feat of engineering. SO and I enjoyed the BMW Museum very much as well. So many beautiful classic cars even an auto neophyte will find something to ooh and ah over.

Emily

Spend day trip hiking in the Alps. There are some great hikes around Tegernsee, Spitzingsee, or Kochel. The hiking is easy and there are many mountain huts serving delicious and cheap meals.

Take the S-Bahn to Herrsching and walk to Andechs through the beautiful Kien valley. It’s about an hours walk. Either walk back or take the bus if it gets too late.

Take the train to Prien, the steam train to the ferry, and tour Herrenchiemsee palace and then stop at Frauenchiemsee island at a Biergarten.

Rent bikes and ride up to Schleißheim to see the park and the palaces, ride back down through the English Garden, stopping and any of the 5 beer gardens in the park.

Take a Floßfahrt down the Isar river.

Visit the recently opened Egyptian museum in Munich. It’s excellent.

When is your trip? It may coincide with a number of events going on in Munich. The Auer Dult, for example, is 3 times a year.

It's already the end of June so you may not need this, but a couple of things to do that are special are to visit the Franc Marc Museum, in Kochel. It's Franz Marc's house and overlooks a gorgeous lake. You can have lunch on the terrace. A bit farther afield is Eng, in Austria. It's reachable by car and is a beautiful valley with easy hiking trails and a dairy farm where you can have a meal.