Where to travel after Berlin marathon: stick to Germany or do Prague/Vienna?

You can without any risk of being bored spend 7 additional days in Berlin, and still feel that you haven't seen it all.

What the best plan for you is depends a lot on what you want to do. Prague-Vienna is not a bad idea, and if you have the time you can add stops in Dresden and Brno along the way. But Bavaria and the surroundings will also be a memorable trip, there is so much more to Bavaria than Oktoberfest.

Jordan,
I did the Berlin Prague Vienna tour and can recommend it wholeheartedly. A fascinating blend of history, culture and art, I learned so much. I really want to revisit Berlin, a remarkable city.

With a 21 month old and only a week, I would not use valuable time - and energy - to visit too many places. Berlin to Prague, Prague to Vienna are four hour trips not including packing, checking out, checking in, unpacking. I would split my time between 2 cities. Berlin + Munich offers a lot to see in the cities themselves with an opportunity for some nice day trips.

With a child of that age, stick to the areas around Berlin so you don't need to travel so far. Potsdam is gorgeous, Hamburg and Dresden are not far and Berlin itself can keep you busy for days. Check out Lübeck or Quedlinburg too.

Although it has been well-discovered by European tourists, Quedlinburg doesn't figure on many Americans' itineraries. It is a gorgeous town with hundreds of (I think over 1000) historic structures in its cobbled historic center. There's a castle and a nice cathedral with an incredible treasure (Google for the American connection). There's a small art gallery. In pre-pandemic times the tourist office rented an audio guide that would keep you busy for about a full day (including the castle and cathedral).

If the weather cooperates and you want an outdoorsy option for a day or so, you might look into the Spreewald area. There are boat rides on the canals; I'm sure there are also boats available for rent. The terrain seemed very flat; I believe there are lots of walking trails in the area

But it's true that you could stay in Berlin and not run out of things to do, though a lot of what I did was visit museums and historic sites that would mostly mean nothing to your daughter.

Berlin / Prague / Vienna would be vastly different from the trip you previously had in mind.

What kind of stay you want isn't very clear. You have been "super active" on previous trips. Does that mean you want this trip to be similar? And by active do you mean moving from place to place quickly? Or physically active in the outdoors? Or something else?

Quedlinburg has been mentioned a couple of times. It's in the Harz Mountains - an area famous for its hiking opportunities - and not far from similar destinations that lie somewhat off the path of traditional international tourism. Have a look at that option and at others just to the west with the help of the "Deutsche Fachwerkstrasse", a route that includes a large number of old-world towns where the distant past is still visible.

https://www.deutsche-fachwerkstrasse.de/en/Routes_Cities/From-the-River-Elbe-to-the-Harz-Mountains.html

Besides the listed towns on that route, there are others in the same general area you could look into, including the towns of Hameln - Hannoversch Münden - and Goslar.

A week isn't nearly enough to really visit all these places. But I can't imagine having a bad time in this area. It's one of my favorite in Germany.

Another vote for the Harz Mountains. I like this website...

https://www.uncommon-travel-germany.com/harz.html

I would normally say both are great options, but in these times I would think sticking to one country would be easiest.