though I believe you will find more than enough to entertain you in Florence without venturing outside the region of Tuscany :-) ...it is very possible to visit Rome. Even in just one day. You can catch the early morning Freccia Rossa/Argento or Italo a new train service (both are speedy gonzales trains) it is possible to be there by 9 am and then home again intime to catch a pizza at a Florentine Pizza Place.
I have done this several times with family and friends who really just wanted to "see" Rome. It is a packed day, and if you really want to get in to any place be sure you reserve in advance. Rome is not as "walk-able" as Florence is so be sure to wear good shoes and be prepared to dive into the Italian public transport service.