Flew Icelandair from PDX to BER in late August and MUN to PDX this week. Customs leaving and coming back were both a bit less time consuming than my past experiences. Icelandair has very, very short layovers in Iceland if you're not doing a stopover. Mine was 50 minutes on the way back and I barely made it after ending up getting randomly selected to interview with customs agent. My main beef with Icelandair is limited in-flight entertainment options and no complementary food for an international flight - not sure if other airlines are doing the same at this time? Needed to get $130 antigen COVID test before boarding the flight to Iceland (even though just for transiting through the airport) but took advantage of the free antigen testing available in Munich before boarding my flight back to US (it was never checked though you are required to sign a paper that you completed and met the testing requirements). Some folks in front of me at PDX customs did have their covid tests checked, but I didnt.
At places where it was checked - most restaurants, museums, castles/palaces, and other indoor venues - CDC vaccine card worked fine. I clipped mine to the inside of my passport ID page so it was easy to verify and took everywhere with me. Always kept it in the same pocket of my day bag so I wouldn't be prone to misplace it.
Train travel was easy, even during the 5-day DB train worker strike. Stops on my itinerary included Berlin, Hamburg, Cochem, Wurzburg, Nuremburg, Munich and Salzburg. Sticking mostly to one country made understanding the rules pretty easy, though I know rules within Germany can vary by state. I didn't notice any differences between the various German states I visited. Most everyone wore their medical masks indoors and on transit - usually one or two that didn't. Salzburg had far less mask wearing indoors. Virtually no one wears masks outdoors in all the places I went to, which was a relief to me.
Many museums, castles, etc I went to continue to offer audioguides in English and other languages, but just as many don't. Also, English language tours are much more limited, if offered at all. If the RS guidebooks notes that a museum/castle doesn't have good English displays and you find out there's no audioguide or English tour, you may not want to visit (assuming you don't speak the language). If this is a concern, you may want to check website and/or contact the place in advance. For instance, I took the a Hamburg Harbor tour specifically for the English tour, but found out they don't offer the once-a-day English tour these days (despite no info about this on their website).
Some places were probably better to visit this summer than non-pandemic summers due to capacity limits. Berg Eltz in the Moselle River valley was a great time to visit because there were fewer English-speaking tourists and our tour group was about 8-10 people, whereas the RS guidebook says these are usually much larger. Miniatur Wunderland in Hamburg has significantly restricted capacity, but in my time there, I can't imagine being packed in there with any more people! The result is that some of these places can be difficult to get into without advanced reservations, but if you do that, you'll probably get a better than usual experience like I did.
Many of the best travel experiences in my humble opinion are completely unaffected by the pandemic, such as hiking through a foggy forest morning to Berg Eltz, watching the sunset and listening to the church bells from viewpoints above Salzburg, and biking along Hellbrunn Allee (and many other places in other cities). Parks are all open and being used by locals to play beach volleyball, table tennis, sunbathing, and everything else. This trip was definitely worth the extra bit of uncertainty and flexibility.
Hope this helps inform some of you considering whether to make a trip...