Best stops between Florence and Venice

Padova is lovely and many here like Verona. They could establish a base in one and visit the other. Each is worth a full day of touring. Both are small with train stations close to the center. In Padova's case, there is a shuttle bus. Both are well-covered in the R.S. Italy guide.

I agree with Laurel, and would add two other places for consideration.

Bologna is just across the Apennines from Florence, has a remodelled station, lots of interesting arcades - covered porticos - which come in handy in the summer heat, a university which is walked past on the way into the centre, and fabulous food.

Ravenna is off the main line a bit, Florence > Bologna > Ferrara > Ravenna. Continuing to Venice is Ravenna > Ferrara > Padova > Venezia Santa Lucia.
World class, or class defining mosaics at several locations, all of which are easily walkable and a passably fine town.

I'd also recommend Ferrara but I don't yet know it.

I love Padova, but you could easily have it as a half-day trip from Venice: it's only half an hour away.
Instead I'd suggest to look into Bologna. It has a marvelous medieval centre, quite a few interesting things, and good restaurants. Moreover, it is an excellent base for historical cities around, such as Ravenna, Ferrara and Modena, all easily reached by frequent trains.
Bologna is a little bit over half an hour from Florence, and two hours from Venice on the cheap Regionale train (1,5 hrs with the more expensive high-speed trains).
(It seems I'm not the only one in favour of Bologna.)

I'm in the Bologna group. They could stay for the three days and still not see it all. Verona is a quick day trip too.

Another vote for Bologna! It is completely different from either Florence or Venice, so you will have a more varied experience. And, the food!!! Also, I felt very safe walking around late at night.