I wish you would explain some more about your motivations etc for this trip. You say you are "planning a trip to Europe starting in Bulgaria." I am assuming from your use of the word "Europe" that you are Americans or Canadians and you plan to travel to Bulgaria then travel other places? I guess I am asking because the seaside of Bulgaria these days is mostly wall to wall, all-inclusive package tourists for low-income travelers from the UK, Netherlands, Russia and Germany. It is very unusual for Americans to travel specifically to vacation there, and you could be disappointed if you didn't know what to expect. I guess it would be like a German wanting a beach vacation to inquire about traveling to Panama City Florida or something. A German looking for some nice beach resort would not go there, same thing as the seaside in Bulgaria, there are much better seaside places in Europe.
That said if your heart is set on Bulgaria, the TA forum for Sunny Beach would be the place to ask this question. There are a bunch of British people there that know all the different resorts inside and out and would be able to better advise you specifically on places to go.