Jayhawk, i regret to say that due to my opinion none of the ports or cities you're going to visit is recommendable for biking. Sightseeing in Nice (or Monaco respectively), Rome, Naples is best done by tour, or, if you're a strong hiker, on your own with a good guidebook and map. For Marseilles i'd recommend a tour anyway as the city is nothing special.
For Tuscany, i'd suggest a train ride to Pisa or Florence if you're out for the most popular sights. There are also trains to Lucca, Pistoia or Prato, if you prefer beautiful, less Touristic cities. If you want to see the typical small towns and landscape of Tuscany, i'd suggest to rent a car and drive to Volterra and San Gimignano (the Manhattan of the Middle Ages). Driving takes its time there but is worth the effort.