We have never rented a car in Europe before having always, trained, bused, biked or hoofed it. But we are considering a one day rental for a day trip outside of Paris and another for a day trip outside of Florence. We want to see some smaller places off the rail lines. I envision us picking up and dropping a car off on the outskirts of town. My questions are: are there rental agencies on the edges of town; how expensive is this likely to be; do we need to arrange in advance before we go; is it worth it to get to see the countryside in a way we can't from the train. Or are we better off sticking to the railways and saving the car for a more countryside oriented trip later? I posted a similar question in To The Boot.
For Paris, there are so many day trips or even overnighters that can be made by train that it doesn't seem necessary to rent a car just for the day. But if you wanted to do so, I'd guess you could pick up and return the car at Charles de Gaulle airport, since you'd be well away from Paris.
In both cities just pick up at the airport and head out of town. If you follow the map and the signs or a gps you will have no problems. Its really fun to have the freedom of the car to explore areas on your own schedule. You will be able to see far more with a car and part of the fun is seeing the countryside and stopping whenever you please. Have fun! Definitely reserve your car ahead and a one or two day rental can be expensive, but reserve at the cheapest and then keep checking the rates... if they go down just cancel and book the cheapest rate. Most cars are manual, so if you need an automatic you must indicate that and it will probably cost more.