I have been to both Arezzo and Prato. Arezzo is a great little town with an old town, a great piazza, restaurants and often something going on either in the town or in the piazza. It has much less of a tourist vibe than Siena or other towns. Exit the train station in Arezzo and wander through the business/entertainment section of town (more the right as you exit the station) and just slowly wind your way up the hill to the old section. Only the really old section is steep.
Prato has a great looking central piazza area but the day I spend the most time there we hit siesta so I try not to judge it by that. It has a couple of excellent but narrowly focused museums - the great textile museum in an old wool factory and the Museum of Deportation and Resistance. In Prato I would catch a bus outside the train station to get into the parts of town you're interested in.
I'd check the schedule for any local festivals or celebrations around the time you'll be there and maybe you'll get lucky.
Pistoia is another town near Florence that has picked up some buzz as an overlooked town, but I don't have any direct experience.
Chuisi is personally outside of the range I would set for a day trip unless there was something there you really wanted to see.
- Verona is an overlooked town I really like. It's a big city but the old city center core feels much more intimate is mostly pedestrian with medieval buildings with faded murals, piazzas, bridges, shopping, an intact Roman Arena and an arty "cafe culture" feel of a city. It's 1.5hrs on the fast train.
- Bologna is only 35 minutes away and while it is a big city it is certainly worth a visit if you haven't been to the foodie paradise.
- Modena is a little more than an hour taking the fast train to Bologna and changing for Modena. Everyone who has been there talks about how much they love it. It's food focused and a very pretty town looking like a mini-Bologna so definitely on the list for me.
Having too many choices is a great problem to have, enjoy!
=Tod
