Ditto the pickpockets, especially in crowds. They're very talented. We lost our camera and a gazillion pix - lifted out of the zipped case hanging from my shoulder. You can't be too careful.
Highlights for us included the Pantheon, Colosso, Forum, just about any church you wander into (check out the St. Matthew Caravaggios in San Luigi dei Francesi near the Piazza Navona where there's also the Fontana del Quattro Fiumi), any of the catacombs, Michelangelo's statue of Moses (with horns!) at San Pietro in Vincoli, etc etc etc. You can't go wrong just doing the "biggies", indeed that may be all you can fit in, but do try to find some lesser known places= they're all amazing. Odd bits of 2000 years of history all over the place.
You should just resign yourselves to the idea that you will be ripped off in small ways, and not worry about it. Save your energy for seeing as much as possible. Walk as much as you physically are able. Eat margarita pizza from a little shop (typically sold by weight, just eat it as you walk on to the gelatto shop), spaghetti carbonara yum.
We liked the Eyewitness Travel Guide a lot. Rick Steve's guide is also good.
Have a great time - I envy you!
. For that reason, we're not planning on driving.
