One Month, One City Update
 

It was a fascinating thread to read all of your suggestions, just wish we had another 20 years to try a few of them! My husband and I have decided on two cities of two weeks each: Berlin then Prague. Modern to medieval! Husband has always loved WW2 history and when we visited Berlin about 10 years ago we enjoyed it very much. We loved the seamless and clean public transit, all the parks, friendly people and culturally diverse. We also love Prague, its beautiful architecture, smaller feel and the feeling of being back in time--well, a little! We have wanted to return there also. Going back to familiar places we love seems a good stress free choice (hopefully!).
Not renting a car and not packing and unpacking many times by moving around are ways we are eliminating some of the stress. When we were younger it seemed like an adventure but now not so much--we need a more settled vacation. Taking a train to Prague from Berlin, then back again the day before we leave for home was our choice because allows us a flight that is much shorter in hours. Voila! Can hardly wait until September to start!

I was in Prague and Berlin this past Jan/Feb. I would love two weeks in Berlin but as much as I liked Prague a week would be my max.

Have fun!

Your holiday sounds lovely. We live in Vienna, and so are more familiar with events and activities between Prague and home. With two weeks in Prague, in September, there will be wine festivals in many of the villages; and the castles and palaces will still be open. You won't suffer from lack of things to do!

I am in Prague several times a year , just back from a trip there and I visited Berlin last year and really enjoyed it.I think taking the slower paced holiday is so much more fun and more relaxing, certainly in September you will be able to enjoy the outdoor spaces in both cities.If I can be of any help to you in Planning your Prague trip just ask, I am back there in early August leading a group of walkers around some of the lesser trod areas of the city and some a bit further afield.

Just thinking, if your Husband is into WWII then ask him to research Reinhard Heydrich as there is a connection to him in both cities.
In Berlin you can go to the House of the Wansee conference where the "Final Solution" was decided upon, Heydrich chaired this conference and it is relatively easy to get yourself there.
In Prague Heydrich was assassinated and there is a memorial to his assassins in Prague, you could also head out to the village of Lidice (20 minutes on the bus from Nardazni Veleslavin) this is the village that was destroyed after the assassination and there is a beautiful memorial there as well as a very good museum telling the story of the tragedy of Lidice.
Also the Army museum at the bottom of Zizkov hill is very interesting last time I was in there about 6 months ago there was a great exhibition on the resistance movement in Czechoslovakia during WWII.