It depends on what you want to experience.
In my opinion it is very difficult to experience any of these places just by staying there a day and taking a walk along the main turistic street. For the islands, I would recommend you to take at least two-three days and also go inland to see places less exposed to changes due to hotels, restorants etc. Split you can go through in a day, it is enough for a quick tour through the old city.
Evenings in Dubrovnik are lovely, enjoy few evenings there instead of running around, especially if you are young couple, I got very nice memories from the place.