Colin and I were lucky enough to get away for a child free long weekend last month, while his parents took the boys to a caravan we jetted off to Rome. The city itself is absolutely breathtaking, everywhere you look there are amazing sculptures, cute shutters on the windows and quaint little restaurants. We were incredibly lucky with the location of our hotel, we were just around the corner from the Colosseum and walking distance from most of the sights. We actually managed to see most of them in one day, just wandering from place to place (Thank the Lord for Google Maps!) and we were in a prime location for daily naps once we had got tired!
The Trevi Fountain and Colosseum were absolutely incredible, definitely worth the visit, we waited in line for the latter rather than paying for a guided tour and the line didn't take as long as we'd thought. We used the audio guides which were interesting and made a morning of it. We did pay for a tour of The Vatican City sights, which I'm not sure was completely worth it because the line wasn't as long as we'd thought but the guide was lovely and informative.
We spent our days walking miles and miles, eating more carbohydrates than should strictly be allowed (but oh, my, the pizza!) and gazing at the spectacular sights. A beautiful place for an adult holiday, probably too much walking for most children though we did see quite a few families! I would definitely recommend.