The biggest problem with travel isnt the COVID restrictions, it is understanding the COVID restrictions. A few countries have made good efforts to make it easier. Finding the information can still be a bit of a challenge. Magazine articles pop up all the time but the information is dubious and dated. Start here: https://travel.state.gov/content/travel/en/traveladvisories/COVID-19-Country-Specific-Information.html
You will often find a link to an official website. Websites like these:
Ukraine: I love this, they have a website and went through the trouble to make a video: https://visitukraine.today/?utm_source=email_send_to_ua&utm_medium=email&utm_campaign=14.08.2021&utm_source=sendpulse&utm_medium=email&utm_campaign=14.08.2021 and https://www.youtube.com/watch?v=0mkCfx5DR1A
Hungary: Okay, I posted it and some people got confused. I think its pretty clear and direct. Just dont panic and read it slowly. http://www.police.hu/en/content/information-on-general-rules-of-border-crossing
Greece: https://travel.gov.gr/#/
I am sure there are others, and if we could make a list of the official government webpages some might find it useful.