Ooh, yes, agree with YIAM, go to Trastavere if you can, it's really lovely and less 'touristy' than the more central areas. DH and I had a lovely meal there.
Personally, although I was glad to have seen the Spanish Steps and Keats-Shelley Museum this was the busiest of all the places we went to, literally heaving. (the Steps, not the museum, that was deathly quiet!)
With regards to the Colosseum tour that niceglasses mentioned, we took one and it was good but VERY quick (our guide took us round in twenty minutes and you couldn't go back in once the tour was finished) the second part of the tour of Palatine was excellent and the guide was very knowledgeable and informative and he filled in any gaps the Colosseum tourguide left, but I guess all the guides are different. Having seen the queues I'd definitely still agree that it's worth paying extra for the tour especiallly as there are no information signs up to tell you what things are in the Colosseum, (or the rest of Rome, unlike when you visit NT properties etc in the UK where you have placards around telling you what things are) With that in mind, I'd also recomend a comprehensive guidebook. Ours were indispensible (we had a couple of different ones) I'm sure you've already got one but just thought I'd mention it.
Oh, and the ice creams...... can you bring me one back pleeeeaaase?! lol