If you're a U.S. traveler planning to go to Europe in the next couple of months, here's the one piece of advice we can give you that you should pay any attention to: Keep checking official sources, and don't rely on yesterday's answer.
After what amounted to a summer pass for Americans, the European Union has taken a good look at its own situation in combating the Delta variant and it's taken a good look at the disastrous rise in parts of the U.S., and has decided to tighten up—but each country is making its own rules, and even then they change day-to-day.
The most general true statement is that more and more European countries are admitting only fully-vaccinated Americans. The Netherlands and Denmark have gone that way. Sweden is banning all non-essential visits by Americans. Italy and Spain are demanding recent negative Covid tests even for vaccinated American travelers.
Some countries are talking quarantine again, at least for a few days. The Netherlands has already imposed a 10-day quarantine for vaccinated Americans, which can be cut to five days by a negative test. That move means immediate major changes for river cruises that begin in Amsterdam, which is where most Rhine cruises begin or end. The cruise lines will either have to find an alternate port, or guarantee getting passengers out of the Netherlands within the 12-hour transit grace period.
As for official Covid advice from the various countries: the likely best sources for now are the web pages of their embassies and consulates in the U.S.; generally they refer you to the official posted rules on their country's health agency. Also try the web pages of the U.S. Embassy in the country you want to visit; they often report information they have been given by local officials.
But, a last word of warning: even the official sites are a gamble. I'm writing this from France, where the official page that explains the (changed three times in two weeks) procedure for getting your U.S. vaccine record into their system still informs you that if they don't come through in time (they didn't) you can get an antigen test on arrival and it's good for 72 hours. Which it isn't: the window was changed to 48 hours for some purposes over a week ago.