What are some must see places in Italy?

I'm going to Milan, Florence, Venice, Rome, and Assisi next month and I want to see as much as I can. What are the best places to go other than the main things like the Coliseum and stuff?

We had a trip to Venice, Florence and Rome last year.
Our objective was to cover as much as possible in limited possible budget.
Here is the list:
You can check full details at http://travelextracts.com/content/eurotr... where we have posted our day wise plan.
Venice
1. Academy Gallery (Gallerie dell'Accademia)
2. Bridge of Sighs
3. Correr Civic Museum (Museo Civico Correr)
4. Doge's Palace (Palazzo Ducale)
5. Grand Canal
6. Naval History Museum (Museo Storico Navale) & Arsenale
7. Rialto Bridge (Ponte di Rialto)
8. Rialto Market
9. Saint Mark's Basilica (Basilica di San Marco)
10. San Rocco (Scuola of St. Rocco)
11. St. Mark's Square (Piazza San Marco)
Florence
1. Ponte Vecchio
2. Uffizi Gallery (Galleria degli Uffizi)
3. Basilica di Santa Croce
4. Duomo - Cathedral of Santa Maria del Fiore
5. Duomo Museum (Museo dell'Opera del Duomo)
6. Galleria dell'Accademia
7. Giotto's Bell Tower (Campanile di Giotto)
8. Medici Chapels (Cappelle Medicee)
9. Piazza del Duomo
10. Piazza della Signoria
Rome
1. Borghese Gallery (Galleria Borghese)
2. Capitoline Museum (Museo Capitolino)
3. Colosseum (Colosseo)
4. Imperial Forums (Fori Imperiali)
5. Palatine Hill
6. Pantheon
7. Roman Forum (Foro Romano)
8. Spanish Steps (Piazza di Spagna)
9. Trevi Fountain (Fontana di Trevi)
10. Villa Borghese
11. Castel Sant' Angelo
12. Vatican Museums
13. Vatican City
14. Sistine Chapel (Cappella Sistina)
15. St. Peter's Basilica (Basilica di San Pietro)
16. Barberini Palace
17. Circus Maximus (Circo Massimo)
18. Ostia Antica
19. Palatine

You can find more details about each of these here:
Venice: http://travelextracts.com/content/venice...
Florence: http://travelextracts.com/content/floren...
Rome: http://travelextracts.com/content/rome-a...

Also you can read about our experience about staying here:
Venice: http://travelextracts.com/content/venice...
Florence: http://travelextracts.com/content/floren...
Rome: http://travelextracts.com/content/rome-a...

in rome definately go see the Pantheon, Vatican City, Piazza di Navona, Spanish Steps (Piazza di Espagna), and the trevi fountain. In Venice you have to take a gondola ride of course. Also a boat ride out to Burano Island is a must. Murano glass factory is also pretty cool you can get a tour although its expensive to buy glass directly from the factory. Theres not much to see in Assisi other than the church where St Francis is buried and the shops. its a pretty small town but it is beautiful. In florence the statue of David is a MUST.

*MILAN: Book a visit to the Last Supper of Da Vinci, then go to the Cathedral, go to the Brera Gallery.
*FLORENCE: Check the main museums: Uffizi, Academia, Bargello and Pitti Palace. Eat the Ghibellini Ristorante. Enjoy the sunset at Micheangel Terrace. Go to Duomo and Santa Croce Church.
*VENICE:Go to the San Marcos Cathedral, Guggenheim gallery, The Palace of the Dux . Taste the grappa liquor.
*ROME: Visit the Vatican, San Pietro in catene, The Colliseum and the catacumbes (the hiden city).
*ASSIS: Direct to the Cathedral.
Buy an Eurailpass for 1 month , 5 steps in the travel agency near you.
Try the domestic wines, deserts and pastas.
In Rome, pick the Touribus , to have a easy tour.

I'm from Tuscany, so what i can suggest you while in Florence is to visit VINCI, the birhtplace of Leonardo da Vinci, his meuseum and the little medieval villages all around. even if Vinci is known by tourists it is not croweded and you will meet local people living there. it's interesting to visit the countryside, life in the countryside in Tuscany is so different from the city life. other oarts of Tuscany can be really surprising fr its Beauty, things to see and to do. have a look at www.ariannandfriends.com for more info and maybe a logisitc help for visiting Tuscany. enjoy it

while your up north visit Lido de Jessolo with its 10 mile beach and the hotels serve a marvelous evening meal and if you want to pop along to Venice you can hop aboard a boat that takes you right to Marks square//all the places you have mentioned are good but i have never liked Roma but if you don't go there you'll miss out on the Coliseum if time permits Napoli and Sorrento and Isle of Capri and see the Blue Grotto and Gracie's villa if its still there //60 odd years i was there last