If you plan on visiting Rome, especially with a short amount of time, be sure to check out the various hop on- hop off buses. Most are double deck style buses with an open air upper deck. They continuously drive around the city and stop at all of the major attractions (Vatican, Colosseum, Spanish Steps, etc.). The great thing about these, with the exception of the multiple stops, is that a pass lasts for the day. So, if you are like me, you can get off the bus at each stop to enjoy the sights and sounds.
Of course, there are a TON of tourists from all over the world on these buses, so unfortunately, it will not be an intimate moment for you and yours. But, then again, it wouldn't make a difference if you were on the bus or not, you will find that each of these sites are absolutely packed with people.
If you are staying at a hotel, be sure to ask the host if they have a buy one, get one pass. It seems that each of the bus line operators offer a free second ticket with the purchase of the first, but presumably, you will need to get the coupon from a local business.
In short, this is a great way to see the city, especially if you don't have much time. You will hit all of the major sites, plus, you have the freedom to get off the bus at will to explore further.
P.S. Make sure that you sit on the upper deck if possible.......it offers fantastic views of the city's unbelievable architecture. Enjoy!