Galleria Borghese and Borghese Park- the Galleria Borghese is a former Cardinal's palace that sits inside Borghese Park. The Galleria is filled with works of art and sculpture. The most impressive works were the collection of Bernini sculptures. I cannot say that I new much about Bernini sculptures before this visit. But, after I am definitely a fan. This museum took about 2 hours to see. Reservations are required, a it is a rather small museum. Before or after, take some time to walk around Borghese Park, as it is incredibly beautiful.
The Forum - In the afternoon of that same day we visited the Galleria Borghese, we went to the Forum. The Forum is right near the Colliseum, so it is possible to see the outside of the Collisuem and get pictures on your way to the forum. I think the Colliseum is best viewed from the outside, so this works out perfectly. We were absolutely fascinated being inside the Forum. It was incredibly beautiful, and was on of those places where you can almost feel energy from the enormity of the history of the place. Definitely, bring a guide book to help with what you are looking at. We used Sloan's favorite, Rick Steves. The light in Rome is absolutely incredible, you can almost tell in my pictures, as I am in no way a photographer.
The Pantheon - I think the Pantheon is actually open 24 hours. It is an easy place to visit for 15-20 minutes, as you make your way to one of the nearby espresso meccas!
Tour by Taxi - on the list trip to Rome, we made a habit of having our taxi drivers stop at a famous piazza or fountain on the way home to the hotel. This is probably the best tourist tip we have to offer. We visited Piazza Navona, the Trevi Fountain, the Maltese Embassy and the Colliseum this way. Another time, the driver waited while I ran into Tazza D'Oro for an espresso. Every taxi driver we had was incredibly nice, they will all gladly wait a few minutes for you while you run out to take a picture of whatever you are seeing. Something like the Trevi Fountain is best viewed at night with less people around, and really only takes 5 minutes. Even though the fountain was still swarmed by agressive Bengalis hawking trinkets at 11PM in the rain in January. The traffic in Rome is not bad at night either. Be sure to tip, and the driver will be your best friend, as Italians don't.
The Maltese Embassy - This is a little far out, but not far from Trattoria Monti. The embassay is in a beautiful section of town. What makes it famous is the key hole on the gate. If you look through the key hole, it perfectly frames St Peters which is way in the distance. It is a cute oddity. There will be guards an people there at all hours of night.
St Peters and The Sistine Chapel - these are probably both something you need to do if you have never seen them, although they are definitely a production. On our honeymoon, we did go to see the Sistine Chapel, we waited about an hour to get in, and then once inside were hoarded through on a packed march with thousands of others. After walking through a large museum, you slowly trudge through the chapel. It is a shame that is becomes really hard to appreciate. I have read that they do small private tours in the evenings. This is sure to be very expensive, but might be worth it for the real art lover. On this trip, we did not make it into St Peter's because it was way too crowded. If you are visiting both of these sites, plan a whole day.