To see what we did on a Carnival Dream Mediterranean cruise in 2009 see here
Carnival Magic Inaugural Venice Cruise May 1st Homepage is here
Carnival Magic Inaugural Barcelona Cruise May 10th Homepage is here
Day 5
Maybe Underground Naples and/or the town of Sorrento.
Underground Naples: http://travel.nytimes.com/2009/06/14/travel/14explorer.html
http://travel.nytimes.com/2009/06/14/travel/14explorer.html?pagewanted=2
Sorrento: http://www.sorrentotour.it/
Getting to Sorrento:
http://www.sorrento-online.com/english/orari.htm
By train: http://www.sorrento-online.com/english/orari.htm
By hydrofoil (fastest):
We are cordially invited to visit: http://www.limonoro.it/aboutus/index.html
HoHo: http://www.sorrento.city-sightseeing.it/eng/percorsi.htm
HeinBloed has some good information on Barcelona: http://www.heinbloed-on-tour.de/
Tom's Port Guide, Naples: http://www.tomsportguides.com/Naples-04-10-2011.pdf
Ended up doing a couple of the Hop On Hop off bus routes... skip to page 2
