I love Dubrovnik and, though I don't like it nearly so well, I think Split is certainly worth going to for the ruins. Also agree about Trogir. Hvar is lovely, and very trendy, though April's probably off-season for that. Another possibility (and one I very much regret NOT seeing except from the ferry, but I was also limited for time) is Korcula. I did take a day trip from Dubrovnik to Montenegro, and the coastline was gorgeous. All good choices.