Montenegro maybe?

In August I spent some time in Croatia and drove into Montenegro for a few days, then back to Dubrovnik and flew on to Athens. So I guess i did it both ways. The crossings were easy then. Have you checked the most current requirements? Do so, then decide.

Personally I am not a big fan of Greece (especially Athens - did that for my daughter) and would stay in Montenegro. I've been there a half dozen times and will be back late this year. Beautiful country. My only suggestion is that if you are coming from Dubrovnik, maybe have lunch or something in Kotor, but dont waste too much time on this tiny version of Dubrovnik. So, so much more to see.

I would want to be in Athens at least by May 4 if the flight to Santorini is early.
I can't speak to covid and borders, but Montenegro vs Greece, I'd consider the following:
Have you been to Athens before? Do you have time there after Santorini?
I am not much of a city person, but I'd still choose Athens over Montenegro, but if you are really into outdoors activities, it could be nice. I'd choose the mountains over the coast, which will pale after Croatia.
If you are just not a city person at all, then you could pick up a car upon arriving in Athens and drive to Nafplio, which would be a great stop for that time frame.

valadelphia; I will be in Montenegro this summer. Tell me the places that let you down so maybe I avoid them.

Hi James,
I believe you have a lot of experience in Croatia, yet? Will it be your first trip to Montenegro? I enjoyed my time in the bay of Kotor, and it is a beautiful area, but Croatia is just my complete ideal of Mediterranean beauty. So, my comparison for leeaylor was more based on their short time frame, etc. It was more of a hunch and I'd really need to know more about their interests and plans in Greece to give the best advice.
I think Montenegro will be more satisfying to someone interested in history, and it sounds like the mountainous areas are wildly beautiful. I did not get south of Budva. Budva has some historic structures of interest--I recall finding the library fascinating, but it was fairly crowded and just did not grab me like Croatia has (four times and I'd retire there in a heartbeat). I hope to return one day and perhaps explore the inland, and drive that crazy road to Podgorica. The far south near Ulcinj has caught my interest as well.
I look forward to hearing about your trip!

valadelphia; i spent a few days in Croatia in August last year. The usual, Zagreb, Split, Plitvice Lakes, Dubrovnik. Zagreb was sort of nice. Split was much less than anticipated, Plitvice Lakes was interesting and Dubrovnik is beautiful, worth the effort, but too plastic for more than a day and a half (actually my second trip there, but not by choice).

In Montenegro Herceg Novi was a little more interesting than Zagreb, Perast much more enjoyable than Split; but Kotor most will find a disappointment after Dubrovnik. Cetinje ........ehhhhhh. The road from Kotor to Cetinje over the mountain well worth the effort for the views, thrills and the Prosciutto and cheese along the way. Ostrog Monastery pretty amazing and the restaurant on the edge of the mountain down below the monastery sort of a must do. Budva, good for a night or two; and a good place to do some deep water fishing from. Stari Bar very unique and a great stop or overnight on the way to Ulcinj. Ulcinj a great place to see cultures clash and to imagine something akin to Dubrovnik with less restoration and more "real". Tara River Canyon, pretty spectacular and the countryside amazing. Overall excellent fly fishing country; but I think BiH beats it a bit in that category.