What a great way to celebrate your retirement! I haven't spent much time around Berlin or Frankfurt, but I return to Munich most years and love it.
For accommodation, I love to stay in Lehel. It's walking distance to most of the tourist things you'd want to see, full of interesting bakeries, cafes, and restaurants (popular ones! Reservations are important even for neighbourhood pubs at dinner), and close to the English Garden. The transit connections are great, with trams lines and the U-bahn. I've stayed at both the Splendid-Dollmann (can be quite expensive) and Hotel Opera and enjoyed them both. Hotel Opera was less elegant but on a quieter street. I love spending time walking around and enjoying the shops and cafes of Haidhausen but haven't found a hotel I like as much in that area yet.
For day trips:
- Chiemsee is easy to reach and should be beautiful at that time of year. Why not visit Ludwig II's attempt to recreate Versailles at Herrenchiemsee?
- Pop over the border to Kufstein in Austria which is the perfect size and distance for an enjoyable daytrip. The fortress is very interesting to visit and the town is charming (plus it has many opportunities for that most delicious of Austrian sweets, topfenstrudel, with or without vanilla sauce)
- Salzburg, if you've not been before, is always great though I find the summer volumes of tourists a bit overwhelming
In Munich, a few of my favourite things are:
- Wandering through the park at Nymphenburg Palace, then into the beautiful surrounding neighbourhood, and finishing at the beer garden/restaurant in the Hirschgarten
- the Sudetendeutsches Museum. It's quite new, wonderfully done, and looks at the history of German-speaking peoples in the Czech lands, right through to their expulsion at the end of WWII (when many of them resettled in Bavaria)
- Watching the surfers in the English Garden. Like small children, I never get bored of this (also, generally wandering around the park)
- Visiting the Munich Readery, a fantastically well-stocked English-language bookstore (most appreciated on long trips, when I've gone months without seeing English bookstores)
One of the most useful books I've found for Munich is the Monocle Travel Guide to Munich (from 2017, I think?) because of the suggested neighbourhood walks it includes. These outline not just buildings and places of interesting but also good cafes and restaurants to visit - perfect for a slow exploration of the city.
Have a marvellous trip!