Barcelona
A beach, a mountain and Gaudí, all reachable on foot from a single apartment.
A Roman palace you can live inside, and a ferry terminal to a dozen islands.
Split's old town is literally built into the walls of Diocletian's palace — bars, apartments and market stalls occupying fourth-century Roman rooms. That alone justifies a couple of days, but the real reason to base here is the ferry port: Hvar, Brač, Vis and Korčula are all reachable in a morning.
It works best as a hub-and-spoke trip. Two nights in Split, three or four on islands, and a day inland for the waterfalls at Krka.
The sea is warm, the ferries run a full schedule, and the old town isn't shoulder-to-shoulder the way it is in August.
A workable shape rather than a schedule to obey — two fixed things a day, and room to change your mind.
Jadrolinija and Krilo catamarans handle the islands — book ahead in season, and note that foot passengers and cars are separate queues. A car is useful for Krka and the coast road, and a liability inside Split's old town.
Island-hopping with a group is where plans quietly fall apart: ferries are limited, they sell out, and one missed sailing costs a day. Put every departure time on one shared itinerary and give the ferry bookings to a single person.