I don't have many suggestions for Barcelona.
Many pictures from a Carnival Freedom Med cruise and a more recent Carnival Dream Med cruise are here:
Zydecocruiser Carnival Freedom Inaugural Site Map
Carnival Dream Mediterranean Cruise, Oct 15-Oct 27, 2009
I liked this hotel in Venice - no elevator, not on a canal, but easy to get to. They will run your bags up to your room.
Cą Doge - Venezia
I stayed there prior to the Freedom cruise.
There is an express airport bus that was about 3 euro from the airport to Piazzale Roma in Venice (the end of the road) and the hotel is right there.
We will return on Carnival Magic next spring and you can follow us on that journey!
There are lots of companies that will pick you up at the cruise ship for a day tour of Rome.