Tui gives you very little information on how easy it is to DIY excursions. You can look through all their excursions on the website, and see if you can glean any information about how far the attraction is from the boat.
In Venice, we to a water taxi into town.. The cruise terminal is quiet far, but there is a Vaporetto stop.
Koper, we took ship's coach to Portoroz. The actual port is bleak looking, and we were there on a Sunday when everything was closed.
Sibernik - the tender lets you off in the centre of the town, so if you are not doing any excursions as such, you don't need to get any further transport.
Dubrovnik - the port is out of town, but walkable if you feel the need to stretch your legs. Other than that taxi or bus (the bus was equivalent of a pound when we did it. There's a good supermarket opposite the port if you want to get some local goodies.
Montenegro, you port is right down town, so no need for further transport.
Corfu - we took a taxi into town.