drive through beautiful Chianti
Ciao Ninay,
Ok then - with this added info about your plans, I'll say this: trying to fit in Pisa in about 4-5 hours is a little crazy since it takes an hour to get there and another to get back so that's two hours.... You'll just likely get back to Florence a little later than you plan. It is doable, just expect for you to have little time in Pisa. Also, make sure you communicate with whoever is supposed to meet you at the apartment, there likely is a person who has to hand over keys at a particular time so make sure to set a time later than you think, rather than have them wait there, thinking you're a "no-show" if you're late.
Car rental pick up on Saturday sounds find, just think about parking for those two overnights.
Here's a map (download) with the parking areas around Florence - you can of course park it at piazzale Michelangelo for the night as it is free if it isn't too far from your rental, otherwise find something closer - the fluorescent GREEN lines are the best areas.
Since you have to return to Florence, I actually suggest you skip Montepulciano and the Val d'Orcia region - it is too beautiful to be rushed.
Instead, I suggest you consider driving through
CHIANTI instead, as it is the area between Florence and Siena and is along your route. You can head down into Chianti through the
Via Chiantigiana (SS222) to
Greve, stop there, head on and make stops in
Panzano and
Castellina.
End up in
Siena after lunch, spend the afternoon there and by 7pm make your way to
San Gimignano (maybe a quick stop in
Monteriggioni, a beautiful walled castle along the road from Siena to Poggibonsi). You will still see beautiful countryside, charming hilltop villages and get to see Siena and San Gimignano. Take the roads I'm indicating, not the "fast" Firenze-Siena road way so that you can stop at the small towns if you feel like it. Colle Val d'Elsa and Staggia are also charming, but depending on time, which I don't think you'll have much of, you can see where to stop or not.
As for lunch - all towns have small shops where you can just order sandwiches, called panini, with cold cuts and cheese. So you don't necessarily have to pack anything ahead of time, just make sure to buy between 12-2pm as some places close after... Castellina would be a good place to stop for lunch.