Agree with Jane as to Florence being 'densely packed', which is good for you in that you will get maximum bang for your buck while having a home base. Whether you're a novice or seasoned European traveler, research the possibility of an apartment, seeing that you'll be there 5 nights. We did this last March on our 2nd trip to Florence, and it seemed to help us assimilate in short order.
For daytrips, it would be beneficial to stay relatively close to the Santa Maria Novella train station, and the bus station is right next to that, but anything not too far east, and preferably north of the river would work. It's a walking town.
Yes, Siena by bus is a do, I think Fiesole is a nice little diversion, but I wouldn't plan too much. The magic of Florence is the quiet in the morning when sneaking out for an espresso, or grabbing a quick gelato at night on the way back to your apartment or hotel.
You don't specify the time of year for this trip, which could affect plans from my way of thinking. Hopefully it's not in the heat of summer and tourists, July or August.
Enjoy your planning!