Day trip from Florence

I would go to Siena. Take the bus. You can buy tickets at the bus station that is located just around the corner from the train station. It's a hill top town, interesting and the children will appreciate climbing the tower.

Or go to Lucca and bike the ramparts and climb the Guingini Tower.

I would say that you can't really mess up with doing Lucca OR Siena (as they are both easy and interesting for different reasons). I did Lucca last trip, and this time, when I am traveling with my friend, we are doing Lucca and then a bike tour that includes a bit of the countryside AND Siena.

Lucca is great with the old wall you can ride around on a bike, without committing to a full day of "active outdoors-ness" as you can do a few loops and then start wandering around the town, so you would have VARIED outdoors-ness.

I vote for Lucca and Pisa - one of my very best day trips. I did this in 1989, but it remains one of my best travel memories.

I started by going from Florence to Lucca (by bus, but you can also take the train). Riding a rental bike atop the wall is great, as is walking through the town itself. At least in 1989, it was very little visited (no idea about now).

I then took a bus from Lucca to Pisa, and walked to the Field of Miracles (where the Leaning Tower, Duomo, and Baptistry are). Pisa, of course, is not little visited. But by getting there in the late afternoon, the last tour buses were leaving, the souvenir vendors were literally packing up, and the marble looked lovely in the afternoon light against the grass. I then took a train back to Florence.

Not only is this a great day, but your kids will get to say they saw the Leaning Tower of Pisa - without your having to sacrifice a lot of time for it.

You know your kids best, but I would go with Pisa/Lucca, as Harold suggested. Kids love the leaning tower, and Lucca has lots of outside area for kids to bike ride and run around, spending their pent up energy.