We used BOGO to Europe twice. Good news is that we really saved half and we made the cruise. First time we had decent routing. This summer we had Baltimore - Chicago - Munich (missed the connection) - Venice. Home was Venice - Philadelphia - Charlotte - Baltimore.
I am thinking the same thing as some other posters. Take a taxi to the hotel and even if your room isn't ready, they will hold your luggage. Then you can do Runnerbean or take a taxi to one of the other sites (Sagrada Familia, Parc Guell, Gaudi houses).
In Dubrovnik, just walk the wall (35 E) and then take in the scenery. If you get to the harbor, not a long walk there are plenty of vendors selling things to do. In Split, the NCL tour that includes Trogir is very nice.
Venice both days unless you have been before. That said, last month we spent two days in Trieste and it was beautiful (but not Venice if you have never been)