Renting a car in Venice

I'd take the train from Venice to Florence, and just rent a car to see the Tuscan countryside.

will drive around the city Your question is a little confusing... you do know you can't drive around Venice, right, because of all the water in the streets? Mark Twain cabled, "Arrived Venice. Streets flooded. Please advise."

you don't want a car for Venice or Florence
the train can get you to Florence in 2 hours, why fool with a car?

My note is VERY confusing and for that I apologize! After we tour Venice for a couple days we're heading south to Siena (bypassing Florence; driving around Florence; saving it for a return trip north). My question is do we rent a car at the Marco Polo Airport? Since posting my question this morning I've already learned of our plan from my mom (who's travelling with us) so there's really no question to ask. Sorry to have caused such a stir but at least you've had a chance to talk about Italy again (which isn't a bad thing, right?) Ciao!
Anne

Arrange to pick it up at the Piazzale Roma in Venice. From there, you simply follow the signs to the bridge across the lagoon and onto the mainland Autostrade.

Another alternative is to take the shuttle train across the causeway and into Meastre, where there are several car places, including Hertz, by the train station. It should be a bit cheaper than getting it in Venice proper and has pretty direct access to the highways west. That's what we're doing in June.

As a general rule, renting a car at an airport is the most expensive way to do it.