Playplay - I looked at the route from Florence to C. on mappy. It would give you the opportunity to stop in some very lovely tuscan towns on the way. The ones closer to the start of your trip would be Pienza, Montepulciano, and Montalchino. Pienza would be my choice. Those areas are off of the autostrada headed south of Arezzo (SE)

You could go down the S222 through Chianti to Siena. This might take longer as its not a highway but would give you a view of Chianti. I think the trip above is better though for what you are wanting to do.

I also was only able to spend one overnight in Florence on my first trip there. We arrived mid day and left the next morning. While I was not able to fully do it justice (museums etc), we walked and walked all over Florence and loved it. You can cover a lot of ground on foot. You can see the major sites and decide if you want to return some day.

So, since you have done so much planning and checking to make this happen, I think you can definitely plan a short but lovely trip on your way back to the boat.