The answer is... not really. We did this (walking) but we took the subway to the Vatican station (walk about a mile to the Vatican) saw that. Then walked to the Trevi fountain which was a LONG walk (but we are walkers so except for the length it was no big deal) But you need a good map and the ability to read it. There is another subway stop near the fountain so we ended up getting back on the subway and heading back. (We had seen the Colosseum on embarkation day.)
IF we were to do it over again we would do it this way- Buy the all day multiple trip subway/bus pass. Take the subway to the Vatican, walk to the Vatican, walk back to the same subway stop, go to the Trevi fountain, back on the subway and then go to the Colosseum which is right outside its subway stop ... you should be able to see them all as long as you only spend a couple of hours at each...
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007