Lisbon in Portugal is amazing! I preferred it to Dubrovnik and it’s so pretty with lots to see and do. It has the quaint town feel even though it’s a city and there’s lovely big beaches very nearby too.
I think most of Croatia isn’t as busy as Dubrovnik, except perhaps Spilt and Hvar. We went in late June/early July, which obviously isn’t our school holidays (although lots of other kids around) but didn’t find it too busy. We went to Korcula and Vis which were both lovely and fairly quietl. Dubrovnik, Hvar and Split were all pretty busy but manageable and not unpleasantly so. I don’t know how much worse it would get in the school holidays though. Further North and the small islands are probably the way to go. Ferry travel was dead easy - we booked them all in advance on the internet, and didn’t need a car at all. Also, we stayed in a hotel about a mile or so from Dubrovnik old town, which had a private beach - so we mixed our time with walking around the old town and relaxing in relative peace and quiet - it was a good mix and as we often did the walk twice a day, plenty of exercise, although there were also frequent buses. Serbia looks beautiful from pictures I’ve seen of others holidays but I’ve not been there myself.
Like others said, almost everywhere isn’t as busy as Dubrovnik. In the summer, there are cruise ships every day. Korcula, Brac, Krk, Zadar, or Istria area are all great. Even Hvar isn’t as busy as Dubrovnik. I like Split too - much prefer it to Dubrovnik.