Hi Scott!
It would be helpful to have an idea where you're planning to go in Tuscany? Depending on what you want to do, It isn't impossible at all to explore a week's worth and MORE of the region without a car. Just as a for-instance, there is a lot to see just in Florence, if you've never been there and are interested in what it has to offer. Fiesole is just 20 minutes or so away by city bus, and Siena is about 75 - 90 minutes away by regional bus or train.
Lucca is another very popular day-trip from Florence: abt. 75 minutes by train. Then there's Pistoia (about 35 minutes by train), and Pisa (abt. an hour by train) and, well, you get the idea! Some folks are fine just basing in Florence and day-tripping as the spirit moves 'em - it can be nice for reducing the amount of packing and unpacking - but others might prefer to split their time: 3-4 days in Florence and a few in Siena and/or Lucca.
If you MUST fly both in and out of Milan (Malpensa: abt. an hour outside of rhe city), I'd recommend the Milan>Venice> Florence/Tuscany> Varenna> Milan route. I'd also recommend adding a last day in Milan itself: you'd have a nice half day if coming from Varenna, there's plenty to see to fill those last hours, and you'll be closer to transport to the airport. Our flight back to the states from Malpensa left early enough that we had to be up and out before dawn but that might not be your case.
Have you explored flying into Venice and out of Milan?