Caroline, in Barcelona, one of the best hotels to stay in is the Hotel Colon. It is across from a beautiful cathedral and easy walking in any direction. Not far from Las Rambles.
http://www.hotelcolon.es/eng/hotel/index.asp This link will explain more. Cut and paste it into your address. Las Rambles wakes up around 10pm. Everything is later in Spain. It's where all the shops and cafes are. We took a similar cruise to the one you're taking and we went a two days early so we could really enjoy Barcelona. It soon became of of my favorite places.
In Capri, don't miss The Blue Grotto, as I mentioned in another post. We were in Venice for 2 days but stayed on the ship. The cruise we took ended up back in Barcelona. Are you going to Monte Carlo too? Let me know if you are. I don't see that you have it listed. Oh, you will love pulling into Venice. It's beyond romantic. Be aware that everything is so much more expensive than it was 10 years ago. We had been to a few of these countries before, but this last year, we were really surprised at the prices!