I live and breathe Italy! Even after all of these years back, stateside.
Often the questions are the same so I figured it might be worth setting down some basic suggestions.
(For something similar on Florence, you can go here).
Given the topic and potential permutations of a trip to Italy, this post is scandalously brief and glossing over all kinds of amazing Italian cities and destinations. Also: light on detail about the specific cities and regions I recommend but I figure this can be your starting point. I'll always be happy to add info later if the same question keeps coming up!
Skip down to the bottom for the real skinny but in the meantime here are the answers to "the usual suspects" for Italy trip-planning purposes:
What to do in Florence
Is it enough to say walk around and marvel? Probably not. I'll assume you're going to hit all the biggies (the Uffizi, for example, the shops on Via Tornabuoni, the glory of Piazza della Signoria and Piazza della Repubblica) and suggest you also roam around the Oltrarno section.
You'll find other ideas in my quick-hit guide to Florence.
Tuscan countryside excursion?
I usually suggest Lucca because of all the cities in Tuscany, it is truly unique. It's a walled city, for Chrissakes! There is a piazza that is a perfect oval. And a Medieval tower topped by a tree.
Obviously, Siena (where I spent a semester, studying abroad) is gorgeous as is Volterra. But honestly that's just scratching the surface. There are all kinds of other delightful little cities and no I don't mean San Gimignano.
When in Rome
I personally think you can do just about anything in Rome and you will quickly understand why it is the Eternal City. Just walk around Piazza Navona or Campi dei Fiori. Look up. Eat (especially in Trastevere or Testaccio). Drink. Spend an ungodly sum at the store with the Vespa gear (ahem).
Or better yet, here's my specific advice: Stand in front of the Pantheon, then go in. You could easily fly back home after that. Leave it to the Romans to know the only way to top (literally and figuratively) a gorgeous building with a cupola is to insert a window on the sky.
Seriously, Rome seems to lend itself to any kind of trip. I spent a mere 24 hours in Rome last year and was so thoroughly enchanted, I can't stop thinking about the visit. Needless to say, three days in Rome would be great, as would a week.
More information on Rome here.
What about Venice?
Yeah, what about it? Okay so I am not an expert on Venice or even a frequent visitor. I believe I've been there twice. It's stunning because it's unique but overrun. And I don't mean overrun and hence I judge it and everyone who goes there. I mean overrun, as in, you might not enjoy it that much because it is a tiny city thoroughly overwhelmed by tourism.
That said, the streets are made of WATER. Hard to beat that as a concept. Also the Peggy Guggenheim Museum is quite interesting. Plus if you go, you can read John Berendt's City of Falling Angels as trip prep.
Maybe try Bologna. It's a small, centrally-located city with gorgeous porticos on just about every building. Oh and the food is insanely good. About an hour by train from Florence so you could easily make it a nice day trip. It's in Emilia-Romagna, the so-called "bread basket" of Italy. And you'll be eating a lot of bread there. And pasta.
Wildcard #2 (MILAN)
In fact, my favorite Italian painting (Rissa in Galleria by Umberto Boccioni) hangs in Milan's Pinacoteca di Brera museum (there's also another great painting, ahem -- Da Vinci's The Last Supper. But good luck getting a ticket!).
Other absolutely breath-taking places to visit: the Galleria Vittorio Emanuele, which is near the Duomo (stunning) and La Scala. The area called the Navigli is chock full of cool restaurants and bars, which are canal-side. Have a glass of Oltrepo Pavese for me!
Le Cinque Terre/Italian Riviera
I would be remiss if I didn't mention Le Cinque Terre/Italian Riviera, although I only visited once and it was years ago. The five little cliffside towns are overrun with tourists but also stunning. This description applies to Vernazza, in particular -- the most picturesque of the five villages. You may want to make your base another town in the region -- like Lerici or Portovenere -- but either way, nothing beats traveling by train to these tiny towns, watching the Mediterranean streak by from the train windows as you traverse vertiginous tunnels that seem perched on the side of the cliff and ready to tumble into the sea...and then hiking around the region (also grab a pesto pizza and a bottle of Schiaccetra').
Head down to Southern Italy. The food is so good you will wonder why you didn't come sooner. Most incredible region I've visited. In particular I would suggest Puglia. You can divide the province into three parts, choosing one to visit or as I did, sampling all three: the Salento peninsula, the Gargano promontory, and the area around Taranto. Oh and take along Italy's version of the Michelin guide (the Gambero Rosso) -- you'd be surprised how many entries show up for places like Bari (but you won't be spending a lot because everything is cheaper down South).
In the Salento peninsula, you'll find two types of beaches: spiaggie rocciose and spiaggie sabbiose (rocky vs. sandy beaches), you may eat horse (I did), you'll drink the local wine (the only wines I drink now, including primitivo and negroamaro) and you'll never miss Tuscany's tourists.
Click here for where to eat in Trani, and what to do and eat in Lecce, -- a regional capital that is sometimes called the Florence of the South.
Hit me up with questions in the comments or on Facebook (where I'll be posting this).
Oh and buon viaggio!