We travel about the opposite of you, we rent an apartment and try to live like locals. Other than gathering food at markets and restaurants, we see very little of the normal tourist sites. If we like it, and did not see it all, we just plan to come back. We are 40 and 45 so not quite the retired age, but we enjoy traveling slow
We went to Munich twice last year, once in May and again in December. Instead of the cafe culture like the rest of Europe, Munich has a beer garden culture. Every place you find a gathering of chestnut trees, you will find a beer garden. We really enjoyed just wandering, and trying all the food. Not the best place for a vegetarian, but if you like pork, your will be in heaven. We went in December to sample the Christkindlmarkt's which were everywhere. A return visit has been booked for this December as well. The thing i really liked about Munich, is, it is very clean and organized, very German that way, vs the chaotic feel of Rome. The local economy is doing very well, there seems to be little to no poverty, people are very polite and quiet, and if you are a German car person, you will enjoy the scenery. As far as historical spots, Dachau is about a 1 hour train ride away. Most people in Munich will speak English, I try to communicate in German as much as I can, but after a while, they will let you off the hook so to speak.
We spent a week in Berlin a couple of years ago. Berlin feels very modern, a little cold but hipper than Munich. Berlin seemed to be the opposite of Munich, there is nothing Bavarian about it. The sheer amount of ethnic restaurants in Berlin is amazing, you can get pretty well anything you want, other than German food. There are some very interesting historical parts of Berlin, walking through the Brandenburg Gate into the old East Berlin. The Kaiser Wilhelm Memorial Church is very powerful, you can gain an appreciation of the sheer destruction of WWII. My profile photo is actually an Ampelmann which is what is shown on the walk signs in Berlin. It was originally an East Berlin thing, but the majority of the city has adopted it. I did find that very few people in Berlin would speak English, but that is part of the fun.
If you have any questions, please email or pm me through the forum. We have travelled to quite a few cities in Europe, and many of them several times, and honestly Munich is my favourite city.