I would love to see Lake Como, Venice, Cinque Terra, Florence, Sienna and Tuscany before finishing up in Rome. Any suggestions on how to spend 21-24 days seeing these sights?
I'm glad to see that you're going to have enough time in this trip to enjoy these places. Rome deserves at least four of your days.
What time of year are you going? That will influence how much time you want to spend in each place.
As for the CT, I have to disagree with Kent about Monterosso. (No offense, Kent.) I prefer it to Vernazza as your base (most tourists stay in one of those two towns). Monterosso has a bigger beach and the flatter layout makes it easier to get around in. (I'm not sure why Ken thinks it's overdeveloped. Development is controlled by the government, since the CT are actually a national park.) But Vernazza is certainly enjoyable, if a little cramped feeling.
P.S. Two minor points:
"Please" in Italian is "Per favore", not "Por favore".
And it's "Cinque Terre" not "Cinque Terra". It means "Five Lands" and "terre" is the plural of "terra". The Italians will appreciate those little things. :)
I stayed in Venice, Florence and Rome and sight-seed along the way. THEN I went to Cinque Terra and REALLY enjoyed the laid back atmosphere. It was like comparing Maui to Waikiki. When I return to Italy, my first choice of a place to return to would be Cinque Terra. Don't miss it. Aloha
thinking about staying 3 nights...enough?
Another comment about the Cinque Terre - I stayed in Riomaggiore and had a great time. It's a bit smaller than the other towns, but has a fairly good selection of lodgings. You might also consider Manarola - that's where the "official" C.T. Hostel is located (lockout during the day) and have a look at www.torrettas.com as it looks like an an interesting place.
Regarding your Itinerary, I believe it's possible to see all the locations you listed, however with only 21-24 days careful planning (especially with transit between locations) is essential! I covered just about the same route last year (although I visited C.T. after Rome).