traveling by train from Venice to Sienna. Should we day trip in Florence?

I guess either city will be equally hot in the afternoon. Whether you choose a half-day in Florence on the way in or getting up early for a longer day-trip later, the key to visiting the Accademia and Uffizzi museums is to reserve entrance times in advance at http://www.b-ticket.com/b-ticket/uffizi/default.aspx. The "skip the line" Firenze Card is too expensive (€72) for a short visit.

Why not do both? Or at least make the afternoon visit, and use it to help decide whether to go back later.

I timed my connection so that I had a 4 hour stop in Florence en route from Umbria to Venice in mid-July 2 years ago. It was very hot and somewhat crowded, but I knew where I was going and what I wanted to see so it was a worthwhile stop. The baggage drop is very efficient, but leave time in case there is a line-up. Also, when leaving the station, be sure to take the underground pedestrian passage to go towards central Florence. The train station is fairly close to the centre so a 4 hour stopover will allow you to see a fair bit. However, you won't have time to visit any major museums. I had just enough time to wander a bit, do a bit of shopping and grab lunch. (I had already visited many museums on previous visits, although I would like to visit them again!) I agree with the previous reply: why not? You can get an idea if you want to go back...and you will! Enjoy!

It is just over 2 hours by train from Venice to Florence, so if you leave early you can spend the entire afternoon exploring Florence. However, to include time at the Accademia (for David) or the Uffizi, Florence requires a whole day (at least). It really depends if you want to visit these two museums - best pre booked as well.

There are also plenty of smaller places to see great art, that do not require reservations: the Museo San Marco, the Bargello, etc.

Thank you all so much for the quick responses. Based on your comments we will head to Florence and stay for about 4 or 5 hours before we hop a train to Siena. Any suggestions on lunch and one place to visit?

When you get off your train from Venice, turn left at the station concourse and head to platform 16 for luggage store, which is a manned office.
Take a taxi from the station to Pizzale Michelangelo for a view over Florence. Walk to the Duomo, crossing the Ponte Vecchio, and perhaps stopping for a coffee to soak in the atmosphere on Piazza della Signoria. Depending on the time you have available, consider climbing the campanile, or visiting the Medici Chapels, or perhaps even the Bargello, before heading on to Siena.