I flew ORD-MUC-BRU two weeks ago, with an intentional 12-hour layover in Munich. I had no issues at passport control in MUC; just needed to show my paper CDC vax record along, of course, with my US passport. No testing was required. I am vaxed and boosted.

I took the train to Marienplatz and had a very enjoyable late morning and afternoon. I was asked for my QR code at one restaurant, but was admitted with no issue when I showed them my paper CDC record.

I'm not sure how many "sights" you could see (like the BWM museum or Deutsches Museum), but you have time for a meal and a beer or three and them some wandering.

I had to fill out a passenger locator form for entry into Belgium and listed Germany as one of the countries I had visited in the last two weeks. I do not know what the Greek requirements are.