Daniela,
Could you clarify your agenda for me? As I read this, you are coming into Rome, and renting a car, and then heading to your apartment in the San Giovanni area of Rome.
Then, at some point you are driving from Rome to Pompeii and back on the same day. The following day you are driving from Rome to Pisa/ Florence, and returning that same night? Then a few days later, after staying in the San Giovanni area, you intend to take the car to the airport and turn it in. Is that a correct interpretation of your agenda? If not, please clarify and I'm sure we can give you some feedback.
BTW, parking in the San G. area is primarily all "blue line" which means if you park on the street you have to pay an hourly rate on weekly days... If you're intent on holding onto a car, you might look for a garage with a daily rate.
I have to ask a few questions, also "seeking first to understand" - Why do you need a car in Rome - is it just to get back to the Airport? Have you you ever driven in Rome/Italy/Europe? Have you plotted out the driving distance of a Roma-Pisa-Firenze run? It sounds like you got a great rate... but did you add in "full coverage" insurance - that's usually runs at least 15-20 Euro A DAY and that cost might outweigh the convenience factor if you have the car extra days just to get to the airport....
Just an FYI, diesel has jumped this week to about $8.75 a gallon here in Italy!
If you could clarify, I'm sure you'll get GREAT responses from the board!!!
Ciao from beautiful Rome,
Ron