I lived in Northern Italy for four years (just north of Venice) and enjoyed vacations in Crotia. Some of the thing you will be able to do and see will be determined by your mode of transportation. My faviorate spot is Pula; it is on the beach and has good nightlife. It also has some “Must See” attractions. The Pula Amphitheater which is the best preserved of the remaining Roman amphitheaters. Also, Rovinj is a wonderful day trip. If you have transportation (4 hrs away) the Lukina Jama caves are amazing.
As for your question about Venice…If you have a chance to go…go! But if you want to add Florence and other places just take an Italian Vacation. (Rome, Pompeii, Pisa, Florence, Milan ect.)
In that area of Europe there is so many amazing things to do….
If you have any specific questions just let me know.