The first place that we visited was Vatican City, which was... impressive, to say the least. That's the only word I can think of to describe it.
The next thing we visited was the Trevi Fountain, which was the only thing that I absolutely HAD to see whilst in Rome. And it did not disappoint. I got to have my Lizzie McQuire moment and throw a coin over my shoulder into it, so I was happy.
We also saw the Colosseum, of course. It is impressive now, so I would have loved to have seen it before it was a ruin. We spent a fair few hours there, and I'm still not convinced we saw the entire thing. You can walk pretty much the entire way round.
I will also be returning to Rome one day. I loved it.