Day 1: flight to Venice, then boat to Venice, check into hotel, take a walk around, go to dinner. Try not to go to sleep till 9pm-ish
Day 2 - Full day in Venice: St. Mark's Square, Rialto Bridge, Tour St. Mark's Basilica, Gondola ride for two
Day 3 - Visit the Doge's Palace, visit a glass shop, take a Grand Canal Cruise, Frari Church, Tour the Accademia museum if you like art.
Day 4 - Take boat to Murano Glass Factory if your are so inclined and have time. Check out, travel to Florence.
Day 4 - continued in Florence. Check in, unpack, walk around. Have dinner.
Day 5 - Accademia where you see the David - make sure you get tickets ahead of time. Uffizi Gallery - get tickets ahead of time. Fit in whatever else you can.
Day 6 - Duomo and the Babtistery. Do Rick Steve's Renaissance Walk if you want.
Day 7 - Day trip to Leaning Tower of Pisa
Day 8 - Day trip Tuscany/Siena or in Florence do Bargello, Ponte Vecchio, anything else you missed. I do a search on the web "Top things to see in Venice (or wherever) and add them to my notes.
Day 9 - morning visit somewhere you missed. Pack, check out, travel to Sorrento. Stop in Pompeii if you can for a two hour visit.
Day 9 - Part II in Sorrento. Check in, unpack, walk around, see some sites if you have the energy. Get your bearings.
Day 10 - Day trip to Pompeii and Mt. Vesuvius or trip to Capri and do the Blue Lagoon and the chair lift to Anacapri
Day 11 - If you like to hike do the Path of the Gods hike. Research how to get to the start of it and how to get back to Sorrento
Day 12 - Relax and enjoy Sorrento. Check out, travel to Rome
Day 12 - Part II in Rome. Check-in, un-pack, walk around and get your bearings. Eat gelato! Maybe do the hop on and off bus.
Day 13 - Collosseum (get tickets ahead of time) and Roman Forum next to it. Trevi Fountain and the Spanish Steps if you have time.
Day 14 - Sistine Chapel and St. Peters - Get skip the line tour!
Day 15 - Catacombs of Rome and/or Galleria Borghese and its gardens.
Day 16 - not sure but you'll get lots of suggestions. Eat gelato. :)
Day 17 - Relax, pack, check out, head home. I'm sorry :(