Rome in one evening what should I see?

I'm going to Rome for work this weekend and they didn't offer me any flights options so I really have very little time there. I've never been before and was really hoping for a day to see places but instead I'm going to have from about 5pm by the time I dump my bags.

Is it even worth trying to see anything? Would a walk into the town be enough; just to walk past the Colossium etc? I'm staying nearer The Vatican.