Agree with JP about Split. The city is built into Diocletian’s palace, and is one of the most spectacular and interesting cities you will ever visit. You can definitely do it yourself. Or take a ship walking tour. If you are interested in art, the Mestrovic museum is a short taxi ride from the dock. Plenty to do in Corfu. Again the town is where mist things are located, so can be on your own or on a tour. Koper is a lovely little university town. The old Venetian city is right by the dock, so an easy wander. There is a clock tower to climb. The second time we visited, we did an interesting food tour along the coast and into the hills along the coast. There are also salt pans and a salt spa, and I think there were tours to both of those as well.
