Trip planned for September 10 days

A few of my favourites:

Berlin
Leipzig
Erfurt
Görlitz

Big country!!! I would search the internet and come back when you narrow it down to two or three cities. Stay away from driving. Trains are excellent. Zoo? Same animals in Europe Zoos as US.

Also - always best to book MULTICITY tickets - that way you fly into one city and out of another, so you don't have to circle back. Prices are about the same and you don't waste precious time. Less is more also - don't try to do too much as it takes LOTS of time to travel between cities, check in and out of hotels, etc. The train also makes things VERY easy. Check out seat61.com for great train info

I'd also recommend the Rick Steves book EUROPE THROUGH THE BACK DOOR - you can get it on this site in the store, but it's often cheaper on Amazon. It teaches great tips on trains, No Foreign Transaction Fee Credit Cards, How to get Euros out with your Debit Card, phones, etc.

Save the zoos for America - focus on castles, cathedrals, maybe a concentration camp - do things you can't see here in the USA. You could also look at some of the Rick Steves itineraries and get ideas. Austria is very close too, so don't rule out Vienna or Salzburg if you're close - just don't try to jam in too much. Less is more - savor each town instead of trying to do too much. We LOVE GERMANY. Esp the small/quaint towns - we don't love Frankfurt or Munich as they are big and cosmopolitan, but they are easy to fly in and out of. You could always see Dachau Concentration Camp and do a great Nazi tour in Munich if you fly in or out of that town - PM me if you need a good guide recommendation. Berlin is amazing for the history. But the small towns are where you want to spend most of your time.

Go on Amazon or head to your local bookstore and get the RS book on Germany. Make a list of where you want to go. Then research accommodations.

Some of the best castles are along the Rhein, which is easy to reach from Frankfurt.
Flying into Frankfurt, there are 2 zoos you can visit. The Frankfurt Zoo and the Opel zoo which is in the lovely town of Kronberg, a bit north of Frankfurt. Berlin zoo is famous too, but I haven't been there.

There are hundreds of places to visit around Frankfurt if that is your choice.

I like Rick's books but they focus on a fewer places. For a first view of a country, the Michelin Green Guide is pretty comprehensive and has some maps at the front that shows some ranked choices.

Two of my favorite areas of Germany are near Frankfurt: the Middle Rhine Valley already mentioned and Franconia (Northern Bavaria - Würzburg, Bamberg, Nürnberg & Rothenburg are principle cities.) I also enjoyed Marburg and Friedberg just northeast from Frankfurt. There are castles along the Rhine, and in Würzburg, Nürnberg, Friedberg & Marburg. Those cities are great as well.

We lived in Germany and had our cars for travel. Cars have advantages in rural areas. Now that we are returning for visits, we greatly prefer trains. They deliver you right to the heart of the towns with the minimum of fuss.

Castles on/near the Rhine and Mosel River valleys that are popular for touring:

Marksburg (in Braubach)
Rheinfels (in St. Goar, ruins + museum)
Burg Eltz (not far from Cochem)
Reichsburg (in Cochem)
Ehrenbreitstein Fortress (in Koblenz)

Some castles where visitors often spend a night or more:

Auf Schönburg (in Oberwesel, top-drawer)
Burg Reichenstein (in Trechtingshausen)
Hotel Burg Liebenstein (in Kamp-Bornhofen)
Hostel in Diez (not far from Koblenz on the Lahn River, family rooms)
Hostel in Bacharach
Burg Arras (near Bullay and Alf on the Mosel)

Zoo...
Klotti "zoo" and amusement park (In Klotten, near Cochem - wild animal exhibits, falconry show)

You can see how close all these places are to Frankfurt and to one another on the map that follows. Find Koblenz, where the Rhine and Mosel River form an inverted "V" where they meet up. The red lines out of Koblenz that follow the two rivers are passenger rail lines that connect the towns I have mentioned. It's quite easy and very inexpensive to get from town to town by train.

https://www.vrminfo.de/fileadmin/data/pdf/2017/RLP-Ticket_streckennetz.pdf

Boppard (a good travel base for both rivers) offers all paying guests a free "Guest Ticket" which allows for free rail transport during their stay throughout the green "VRM" zone you see on the map - which includes nearly every town I've mentioned.

Small caveat for staying in castle-hotels: Most all of them are situated high on the cliffsides, the Diez hostel being the exception; train travelers will likely want to taxi uphill with their luggage.

I'd advise you to spend 4-5 nights minimum in the area. What are your travel dates? September is a great month for this area, partly because of the wine festivals that take place every weekend (which of course puts rooms in higher demand, so book accommodations early if you can.)

Rick Steves offers only scant coverage of this region. You should have a look on Tripadvisor and elsewhere for details on the castles and towns in this region. This page may be helpful too. It includes hiking, ferry boat, and cruise boat information as well:

http://www.loreley-info.com/eng/rhein-rhine/walking-hiking.php