First, Dubrovnik: We just did a Med cruise in May that had Dubrovnik as a port. It is totally wonderful, and we did it ourselves. You just get a map and get going after you take the shuttle!
Second: Florence, if you want to see David, definitely get advance reservations at the Acadmia. Ditto for the Uffizi. The best way is to call the museums directly. They charge you four euros more, $10.50 euros instead of $6.50 euros, but you have a definite time. By the way, you do not pay unti lyou pick the tickets up at the box office about 20 minutes before your tour time.
We spent three days in Venice precruise, and toured totally on our own, which was no problem at all. You just buy, in your case, a one day vaporetto pass and get on and off wherever you want.
We spent five days postcruise in Florence. Not sure if you end up there, but it is wonderful, and our best day of the whole trip (8 days land and 7 days cruise) was our day with a fabulous private guide who tooks us throughout the Tuscancy countryside to at least five different towns. If you want particulars, I can give them to you. You can email me at firstname.lastname@example.org
Hope this all helps.