Hotel breakfasts FTW. Saved so much time, especially since this hotel lacked a convenient bakery across the way... that and it turned out to be a good ~15 minute walk to the nearest subway station!
Trevi Fountain was the first stop. Even off-season it's still packed with tourists, and as magnificent as it is, we can only imagine how much more awesome it'd be without a thousand people in front of it. The Pantheon was the same way-- only from the outside, the Pantheon is pretty plain. Inside, though, the architecture reveals itself as truly impressive. The hole in the ceiling letting down light leads to a soft, diffused cast everywhere, the round walls lead to interesting echos, and the evidence of history from Rome's original religious beliefs into Christianity show in subtle ways.
Monumento Vittorio Emanuele II marks the monument of the what would become modern Italy. In an area of thousand year old buildings, this 1920s structure stands out as a massive presence of white stone amidst the ruins. Decent collection inside to modern Italy's founding, although the apparent highlight for many is the view from the roof...
The entrance to the Museo dei Fori Imperiali (the Imperial Forum) next door is an unassuming thing, with the exception of the smartly dressed hosts directing you to the ticket counter and the bag check. For what looks like a single two-level hall of exhibits, it seems expensive. Then you go inside, wander around, don't seem too impressed til you walk outside and realize you get access to the Forum. The Forum of Caesar, Forum of Augustus, Forum of Nerva, and finally the final form-- Forum of Trajan, plus Trajan's Column. It's pretty damned impressive to realize this is the cumulation of almost two thousand years of history in a single spot.
A short walk to the heart of Ancient Rome is the Roman Forum (Latin: Forum Romanum, Italian: Foro Romano). Pay more for admission, but holy crap, it's worth it. It literally encompasses a zillion artifacts and places of significance, including Palatine Hill. Column of Phocas, Arch of Septimius Severus, a few basilicas including Santi Cosma e Demiano, Arch of Titus, Tempo di Antonio e Faustina... then you start walking up Palatine Hill to all the sights up there. Friggin' amazing.
Trudge back down the hill to the Arch of Constatine and then, at last, into the Coliseum! (or Colosseum, depending on how you prefer to spell it) As the site of one of the largest Roman freestanding constructions, the engineering is remarkable, and that's before you get inside and see everything they did-- including the elaborate systems below the central floor! It's large enough inside that some of the hallways have been converted to museums displays themselves...
About the only disappointing thing was that at night, it wasn't as awesome as we expected. Probably need to show up on a holiday or something for them to really light it up? We had (surprisingly decent) dinner at a restaurant a few blocks north in order to see the Coliseum at night, and... yeah. Not that we could complain too much-- it was one long but productive day!