If I were planning this, I would maybe look at 3 things. First, are there any current issues in a location that might make a location less or more enjoyable for my friends. Second, direct flights from Israel. Third, I would want to make it a longer trip for myself since I am coming from so far so is it a place I would enjoy longer or is it a place that is a good stepping off location for something else that interests me.
Budapest while not Balkan is a place where Jewish tourists have no current issues. Its also a very Jewish city if that means anything to your friends. Plenty to see and do and a good jumping off point for a lot of Europe. Summer tourism is easily absorbed by the size of the city. A lot of good hotels depending on your budget and likes. Yes on-stop flights from Israel. I know someone in the area that can probably help.
I love Sarajevo. I am talking to someone about my return in early September. My last trip was May last year and while I don’t recall any issues at the time …. Still worth checking to see if there have been any issues. I like the Hotel Old Town but there are several good ones in the same area. The problem is that there are no non-stop flights, and it might take them 10 hours to get there. You will have to check and see where you can fly to from Sarajevo after your friends leave. Not the best connected city. I know a guide/trip planner/agent in the area that can probably help.
Nessebar is about 45 minutes from Burgas and hits most of your criteria. Stay at the Hotel St. John. Wonderful for about 5 days. Great history and beaches (nearby). Bulgaria still isn’t the “in” place to be so the July tourism more likely to be manageable. Direct flights to Burgas from Israel. From Nessebar after your friends leave, Bulgaria is a pretty interesting country; or Burgas is well connected to Europe with discount airlines. I know a guide/trip planner/agent in the area that can probably help. https://youtu.be/MJFx7R4IgLY?si=F4TuvAmcozbvsj_b
Montenegro. Why not Kotor; the mass of humanity in a city that now only exists to serve tourists. Otherwise, Montenegro is just about my most favorite destination in Europe. I was there for the fourth time in May this year. Kotor is great for a day, but 3 to 5 days, no, not for me at least. I love nearby Perast and it would be very much worth visiting but in July will also be too crowded for me to want to stay in. The boat trips are great (and inexpensive). If you go down the coast things thin out a bit which is why I said Ulcinj. https://youtu.be/LaPcThRQdi4?si=DD75vjwFN3ZsE2Ab Things are better in the high country too, which is why the hotel I suggested above and a couple of road trips. Those trips will cost the group a few hundred dollars for the day. This is an economical country and breath taking in its beauty from high country to the coast. AND direct flights into Tivat from Israel. From Montenegro, after your friends leave you could go on to Bosnia or Croatia or Albania or fly non-stop to Budapest in about an hour, for about $50. I know a guide/trip planner/agent in the area that can probably help.