For BlueBelle and many others liking Barcelona, yes, it's very pretty. BUT - as Sago and maybe others have warned, IT IS RENOWNED (well, maybe not with you?!) for being also a paradise of STREET CRIME!! See the comments on masses of Cruise blogs and other tourist advice sites?
Street muggings, pickpockets, distraction robbery, all sorts. They'll even target hotels when coaches turn up, try and make off with some cases. Police won't be interested if you're a victim, may grudgingly make a note.
Prague is much better than it was, we went in the Iron Curtain days! But I've heard that it's good. Many alternatives, if you like Art, try Vienna, fabulous galleries (and some sumptuous cakes too!). And that 'Third Man' fairground! If you go there, take in Salzburg perhaps, prettiest place we've ever seen.
Lots of alternatives, Copenhagen, Stockholm and Oslo are all good, (depends on your interests) but you need MONEY!
Florence is stunning, and there's plenty in Milan & Turin too (never yet been to Rome). If you're OK with trains, not keen on planes, check the Man in Seat61 website, he details trips to places like Milan which can take in all the snowy Swiss mountain scenery.
Paris is good too, of course, despite the possible drawback of having too many French folk there (?! - sorry!). And pretty easy to get to.
For an off the wall choice, how about Oporto (or is it Porto?) for a day or two, perhaps a Douro river trip too? And if you like Port, then they have oodles of Port lodges to visit with a few samples (IMO, Port there tastes better than here! Hic).
If you're keen on Spain (& generally it's good), we had a lovely cruise, went from Soton and did the ports along the north coast, inc Coruna, Santander and Bilbao, not as hot or busy as the more obvious places further south. Whatever, enjoy!.