Italy is the perfect country to visit if you have four to five days to spare. You can see at least three cities and spend a day to a day and half in each, and if you plan your visits efficiently you can see almost everything one should see when visiting those cities.
Now for a quick and easy to follow check list of things to do in each city:
Venice:
1. If you can, make sure you visit Venice during Carnival. It starts two weeks before Ash Wednesday and ends on Mardi Gras. Carnival began as a way to celebrate and express oneself regardless of class-- by wearing a mask you were able to hide your identity and therefore your social class. Today at least 30,000 visitors come to Venice for Carnival each year.
2. Fork out the money and take a Gondola ride, you won't regret it.
3. Walk through the Piazza San Marco and ignore the urge to feed the pigeons- they will swarm around you if you start to feed them.
4. Visit Accademia Gallery (Galleria dell' Accademia): this museum contains the most important collection of paintings of the Venetian school, with masterpieces by Bellini, Titian, Giorgione, Tintoretto, Veronese, Tiepolo, and Rosalba Carriera.
5. Get lost in the maze of Venetian streets.
6. Share a giant box of Italian pizza with friends for lunch while sitting along the water's edge of one of the many canals.
7. Buy a Carnival mask.
8. Enjoy and marvel at the beautiful Venetian Gothic architecture.

Florence:
1. Visit the Uffizi-- one of the most famous and important art galleries in the world. It contains works by: da Vinci, Donatello, Michelangelo, Raffaello to name a few... oh and it has the largest collection of Botticelli's works in the world.
2. Visit the Galleria dell' Accademia just to see Michelangelo's David.
3. Visit the the Basilica di Santa Maria del Fiore-- it is one of the most important landmarks of Florence and is one Italy's largest churches.
4. Visit the Piazza della Signoria as it is one of the focal points of the city and while you're walking around take note of all the statues lining the square. You'll find a copy of the David as well as two copies of statues by Donatello-- among others with very interesting histories.
5. Enjoy views like this one:

Rome: (I'll refrain from going into too much detail on these since they are such iconic and well known landmarks.
1. The Colseum- prepare for long lines and very annoying tourists.
2. The Vatican and Vatican Museums- the Sistine Chapel is a must see.
3. The Spanish Steps- watch out for crowds, it can get pretty overwhelming.
4. Trevi Fountain- don't miss when you're throwing your penny over your shoulder!
5. The Pantheon- one of the temples dedicated to all the gods of Ancient Rome.
6. Fork out the money for a nice Italian dinner.