Thanks so much for all these suggestions so far! This is really helpful. Keep it coming! I will digest it all and see if I have questions.
Matt, here is the itinerary for our Bavaria/Austria trip, which was really great. At the time the cousins were 3, 7, 8, and 9. Garmisch-Partenkirchen was a good home base for the middle week; we stayed near Ludwigstrasse on the quaint Partenkirchen side. Other days were at hotels. We kept a busy pace but worked in lots of kid-oriented fun (alpine slide, swimming, ice cream almost every day) and a mix of history, scenery, and action, plus pretzels and biergartens. Having a home base also allowed us to time our schedule precisely according to the weather, since Bavaria can be a bit rainy. We rented two cars for the 9 people. Let me know if you have questions! It was a wonderful trip.
Day 1: Arrive Munich, dinner at Hofbraukeller biergarten. Group who arrived the previous day spent the day at Legoland.
Day 2: Walking around Munich (Marienplatz, Peterskirche, Frauenkirche, Hofbrauhaus, English garden, Residenz Palace. We traveled by subway (U-Bahn/S-Bahn) and foot with a stroller for the 3 year old. Dinner at Chinese biergarten in English garden.
Day 3: Deutsches Museum Transportation Center--great for kids. Leave Munich, drive to Garmisch-Partenkirchen. Stop on way at Kloster Andechs (monastery with baroque church/beermaker with scenic biergarten). Climb the dome for nice views. Dinner in Partenkirchen on Ludwigstrasse.
Day 4: Cable car to summit of Zugspitze (highly recommend).
Day 5: Linderhof Castle (elaborately decorated, beautiful grounds), Ettal Monastery, Oberammergau (small town known for painted buildings, woodworking, passion play)
Day 6: Old Olympic Stadium, Partnachklamm (very cool gorge in Garmisch-Partenkirchen, hold your kids' hands tight if it's a wet day!). Swimming at local indoor pool with water slides on rainy afternoon.
Day 7: Mittenwald (pretty small town known for violin making, woodworking), afternoon at the sommerrodelbahn (Alpine slide--kids and adults all loved it).
Day 8: Neuschwanstein and Hohenschwangau castles (classic), picnic lunch at Hohenschwangau.
Day 9: depart house in Garmisch-Partenkirchen for Salzburg. Long day of gorgeous scenic driving via Waldensee, Bad Weissee (stopped for picnic), Bayrischzell, Schliersee, Alpenstrasse to Oberaudorf, through Rosenheim to Salzburg. Dinner in Salzburg.
Day 10: Tour Salzburg. Festung Hohensalzburg, Cathedral, shopping, Mirabell gardens (nice playground here), ice cream, etc.
Day 11: Gaisburg mountain panorama (little known, quick drive to top, excellent views over Salzburg). Day trip to Hallstatt, Austria (highly recommend). Picnic lunch in Hallstatt at beautiful park overlooking the lake near the farthest parking lot (happy accident due to not finding parking close to town right away).
Day 12: Stiegl brewery tour. Drive to Berchtesgaden. Afternoon of eating cakes and swimming in hotel's pools.
Day 13: Boat trip on Konigssee, walk from end of Konigssee to Obersee (highly recommend)
Day 14: Rossfeld Panorama road (lovely but somewhat superfluous after all the other scenic driving we'd done), lunch and shopping in Berchtesgaden, tour of Berchtesgaden salt mines (highly recommend for kids).
Day 15: fly home.