Regarding tour in Rome , its really depends how hot it is outside and how long you want to walk , I can tell you that two month ago we took the train to Termini from there we walked to the famous fountain passing by some churches , it took us 18 minutes walking , from there we went about 5 minutes to the Spanish stairs and to the Pantheon , its another 15 to 20 minutes to the Vatican city. other option will be to take the underground and walk from there (it will save you 15 minutes walking). You can always try to book a car for half day , but you will spend some time on traffic , also the car can not enter everywhere.