We were just in Rome for 3 days. We took a full day bus tour to Pompeii and the Amalfi coast and we're very glad we did. We were tired from the walking tour of Rome and the bus felt great. (We did the Vatican the next day.)
Amalfi and Positano were great; incredible views, exactly like the movies. That's an area we're sure to re-visit.
Pompeii tour was 3 hours, so we saw a lot, but not all of it, which applies to Rome also. We figure you can't see everything in your first trip. We did not see Ostia Antica, so we can't compare. For us, Pompeii was a must-see.
We were picked up early, stopped for breakfast, lunched in Positano, back at the hotel for dinner. We also enjoyed seeing the countryside along the way and the drive through the small towns and along the mountains on 'narrow' roads.
Either way, you're in Italy!! :) --Jerry
PS When we left Rome we drove to Florence for 4 days, but that's another story...