Hi...sorry for the late reply.
I can tell you something about Naples: it depends for how long you're going to be there, there is very interesting spots you can't miss...Pompei, the Vesuvius crater, Sorrento, Capri, Positano, Paestum (one of the biggest and most beautiful ancient cities of the Magna Grecia, recently fully restored)...Rome, you can't miss the ancient spots of the Roman Empire all around Rome centre...Barcelona, the new Marina which is at the end of the ''Ramblas'', the Ramblas, the fresh market which is downtown right near the Ramblas, the ancient Barcelona, the so called ''Born'' which is an area with small restaurants and small fashion boutiques. I've never been in Messina....Venice, you don't need to see anything, you could enjoy it with your eyes closed, it's a magic city, you can feel the enjoyment directly through your skin...

Hope this is enough, if anything please don't spare me.
Have a sweet rest of sunday/Giuseppe