We did a cruise that departed from Barcelona, we got in a couple days early to see the sights. The Gaudi church and Park Guell noted above are both worth the trip (really odd, but incredible).
Love the main streets in Barcelona with the pedestrian walkways down the center, and all the restaurants and bars lined on the sides. Great people watching, fun to explore. Freeking scooters everywhere. Those people are almost as crazy as the scooter suicide riders in Rome. They come within inches of the buses, going 40mph the opposite direction, and act like it's just normal to almost die 50 times a day (which makes sense, because for them it is just a normal daily commute).
Cannes we were just at for the day, we did a bus tour to a small town St. Paul which was about 30 minutes from what I recall. It is hill top and surrounded by a huge wall for protection back in the olden days (before 2 strokes). Pretty cool, not too crowded, and you got to see the French countryside. We also stopped at some perfume factory (my wife and 3 daughters out voted me). But it was actually pretty interesting to see the process. Never knew perfume was actually made from flowers, and the science behind it was amazing.