Italy has 20 regions, specializing in their own cuisine and landscape. I've been able to visit Campania, Lombardy, Lazio, and Veneto. While it's impossible to cover everything, below are some highlights and recommendations I can share after 4 separate visits it Italy.
Tour Guides are extremely helpful, especially when heading to crowded and historical cities. Angel Tours is a great option and made all the difference when visiting the Vatican, Forum, and an afternoon walking tour of Rome. Long lines are inevitable at key locations, so a guide will help you skip to the front as well as offer context for historical sites. If you go during the summer, visit as early in the morning as possible. You'll avoid some intense crowds and heat.
Other good options can be found at Lonely Planet, where you are redirected to a variety of tours with reputable guides.
Apps are helpful shortcuts to viewing timetables and purchasing tickets. If you know where you'll be going ahead of time, download ferries or other option in advance so you're not scrambling in the moment.
Purchase a high-speed train ticket on the Frecciarossa at least a month in advance for a much better price than if you decide to go last minute.
TIP: Be sure to validate your ticket in the orange boxes before boarding the train. Also note that if a train attendant checks your ticket validation, they will not tell you if you're on the wrong train. (I experience this my first day in Lake Como.)
Private cars are also an option, especially if you feel nervous about rounding the narrow curves along the Amalfi Coast. Check out Eduardo Shore Excursions for a local driver in and around Naples, Sorrento, and the Amalfi Coast.
Renting cars give you ultimate flexibility, but opt for a smaller car to save yourself scrapes, bumps, and frustrations. Sedans are often limiting when managing tiny Italian streets and parking garages. Go for the smallest option that will still hold your suitcases.
Summers are best to spend at the beach, not traipsing around cities, but if you must, wear comfortable walking shoes (Birkenstocks are my shoe of choice) and stop often to cool down and people watch. Enjoying a cafe is part of the culture, so relax. You may not see everything, but you'll be rooted in local life and happier in the long-run.
Most Italians agree and simply head to the beaches during this time, so you may find that a good deal of the shops may be closed.
Shoulder seasons -- April - mid June and mid-June through August.
Peak season -- mid June through August
Off season -- November through March
Splitting is perfectly acceptable and necessary at most restaurants. Don't feel the need to order from every course. I'd recommend selecting 2 to 3 courses maximum if you want a variety of options, and split them all. Not all restaurants serve large portions, but most are very generous and don't like taking food back to the kitchen. It feels like an insult to leave food on your plate, so do everyone a favor and start small. There's no shortage of "mangia, manga" in Italy, and you'll enjoy it more if you don't feel completely stuffed at every meal.
Favorites and Recommendations by Region
Mini-Guide: ROME (LAZIO),
Mini-Guide: AMALFI COAST (CAMPANIA)
Full Map Guide: LOMBARDY