The coast. From Berlin, you can get to Usedom in under 3 hours, which has three historic seaside resorts, Ahlbeck, Bansin and Heringsdorf. Gorgeous old seaside hotels and you can spend some time on the beach.
Potsdam is a must - a day trip from Berlin.
I would also add on more time in Bavaria. Go up some mountains. Visit Neuschwanstein and Hohenschwangau castles. The Tegelberg mountain is in that area too and a couple of beautiful lakes so you could easily spend a few days around Füssen.
Yes to the Rhine valley and its castles and vineyards. But it only takes a day or so to ‘do’ it. I’d stay in a castle. The Schönburg in Oberwesel is fantastic. Great food, too. You can then do a boat trip up the Rhine from near there.
For something totally different, a couple of days in a theme park? We love Phantasialand and if you’ve come up the Rhine valley it would be an easy next stop. Or Europapark in the Black Forest.
The Black Forest is worth a visit if you can squeeze it in and Freiburg would be a good base - or Baden-Baden. Both are very pretty.
Dresden in the East is absolutely beautiful.
Then there are lots of beautiful towns and villages with timber-framed houses… Marburg is a beauty. But there are hundreds and a lot of them are in the south so best to consult a guide book!
Are you planning to hire a car? You’ll probably need one to get to the more scenic places. Try to avoid driving south on the motorways during August weekends, everyone will be driving to Italy etc and there are always huge traffic jams.