|Subject: Venice to Florence|
major car rental companies have offices at Piazzale Roma in Venice, close to railway station. It's the square at the end of the bridge linking Venice to mainland. Airport is really offhand if you need it for renting a car only.
If you wish to go to Florence only, I strongly suggest going by train. By road you have to cross the Appennini mountains (highway but slow and often jammed) and parking in Florence is neither easy nor cheap. The train takes about 3 hours and stations are in the very middle of both towns. The price for 2 persons is surely less than one day rental + gas + toll + parking! If you want to go around Tuscany it's a different story of course.