I am trying to choose one or possibly two from these two cities to visit this summer, after I am in Vienna and Budapest? Any thoughts?

Thanks!

I can only help you with Sofia, which I visited for a four day stay a few years ago for a wedding. Really lovely place with lots to see, and nice food too. The guided tour we did with these people was great and very well organised but there are others too, some offering day trips outside the city which sadly we didn't have time for.

https://freesofiatour.com

I'd go to Sofia over Bucharest, every time, though the language is a romance one so relatively easy to pick out meanings in documents etc in Buchareast.

Serbia is still on my boycott list so I've not been.

I have been to all three, although a while back. I would order them: Sofia - Bucharest - Belgrade. Or possibly Bucharest - Sofia - Belgrade. See:

Sofia: https://mytimetotravel.wordpress.com...tour-in-sofia/

Bucharest: https://mytimetotravel.wordpress.com...ing-bucharest/
https://mytimetotravel.wordpress.com...ting-budapest/
https://mytimetotravel.wordpress.com...ell-bucharest/

Belgrade: https://mytimetotravel.wordpress.com...t-impressions/
https://mytimetotravel.wordpress.com...cond-thoughts/

Sofia with day trips.

https://flic.kr/p/HYi1z4 https://flic.kr/p/H3p4AW
https://flic.kr/p/HUY7oW https://flic.kr/p/HRCBP2
https://flic.kr/p/Kjy4oo https://flic.kr/p/HwLK9Q
for the entire album:

https://flic.kr/s/aHskxVhY8m

I also wrote a trip report which can be found by clicking on my name and browsing the trip reports.

lovely photos, Michael. I should love to return to Bulgaria but their vaccine levels are pretty low at present.

Thank you!

Thank you Annhig, Michael, Bilboburgler and Thursdaysd!