if you have been travelling round italy then you will probably have had enough of painted ceilings etc so for me its the colosseum (?) and trevi fountain
use the metro - cheap, cheerful and easy enough to use !
One day, and one day only? Colosseum early in the morning, taxi to St Peters, Vatican museum, finishing in the Sistene Chapel, over to the Spanish Steps for dinner, wander back to your hotel for a nighcap and to plan when you can return.