Best things to do at ports on Mediterranean cruise

My sister and I are both in our upper 20s and we are going on a 7-day Mediterranean cruise in two weeks, stopping in Mallorca; Messina, Italy; Corfu, Greece; and Bar, Montenegro. We are trying to figure out what to do at each port. We want to make the most of each place we go, but don't necessarily want to pay to do excursions for each stop. We are considering a boat tour in Corfu, but not sure if it would be better to save that for when we are in Messina (Sicily). Any advice is appreciated :)