The usual things are the catacombs, the Circus (just a park, now), Trevi fountain, Col, and the vast amount of priceless statuary all around the streets and public areas. It seems you can't walk more then 50 feet without bumping into a Leonardo or a Michelangelo. The two forums are well worth a visit, but I'm sure you've done them. Oh, and Cat central - if you like moggies!