The beautiful, warm, sunny eighty degree weather was a welcome change from the cold and wet weather in London and only slightly cold and slightly less wet weather in Paris. Did you know there are palm trees in Rome? I didn't.
On the itinerary was Vatican City, St. Peter's Basilica, the Colosseum, Roman Forum, Palatine Hill, Ostia, and the Pantheon. It was 3.5 days of running (or metro-ing) as fast as possible from place to place. By the end of this twelve day expedition, I had two shin splints, a partially dislocated hip, and a twisted ankle. No blisters on my feet though (I swear by the Gap's athletic socks).
Anywhere you can get a skip the line tour, buy it before you get there. The line to the Vatican was about a mile long. Especially if you're only staying a few days, it's not worth the hours waiting. Bonus about the tours is sometimes they let you get into places door tickets won't -- I went to every level of the Colosseum from below the stage to the third floor -- and often you get audio tours. If bought at the door, you're not allowed below stage or to the top floors.
In my handy dandy backpack I traveled with my Canon 70D and Canon EF-S 17-55mm f/2.8 IS USM lens. However, by Rome, I was traveling as lightly as possible because of all the walking, climbing, and holy-whoa-amount of steps. For Rome I just took my Canon 70D and Canon EF 50mm f/1.8 II. It's not the best for far away shots and scenes, but it was great in low lights as well as being lightweight. See the shots below for examples.