Slovenia doesn't really have much bouldering anywhere. Osp / Misca Pec sport climbing is really near the coast, but it would be very hot in the summer. Also there isn't actually that much Slovenian coast line and the beaches aren't great.
Croatia is a much better option for beaches and beautiful coast line. Loads of rock and sport climbing / DWS, I've no idea about bouldering though sorry. I've been to the Istria region in the North which is stunning, particularly Premantura south of Pula. Some good 'unknown' sport crags in the area with some info on line if you look around. I don't know about further south, it's a big coastline!
Hope that is of some help
