Tuscany Stay
 

We are planing a trip to Tuscany in mid October. We are excited about driving around and seeing the area, drinking wine, riding bikes, walking the beautiful towns, along with long walks and hikes. Question is; do we stay in multiple places (Florence 3 nights, Siena 2 nights, Lucca 2 nights) or do we find a Agriturismo centrally located (San Gimignano?) and do day trips from there?

You actually do not have much time to explore Tuscany with only 7 nights. This gives you only 6 full days assuming you arrive on the day of night one and leave after night 7. Are you flying in/out of? Coming from and going to? You are also going to lose about a half day each moving from place to place. I do not think you have time for long walks in the country and hikes and bicycle rides if you want to see places like Florence, Siena and Lucca. Consider 7 nights in Florence and day trips to places like Siena, Lucca, San G, Chianti etc. Not by driving. Alternatively stay 3 nights in Florence than move to Siena for 4 nights which is very central to Tuscany and do drive day trips from Siena.

One option to consider
 

When we stayed in Tuscany for 2 weeks we enjoyed Le Tori (Vacation Rentals in Tuscany Villa Le Torri,Holiday Apartments in Chianti,Tuscany ) and would highly recommend it. We were able to take day trips with easy visits to Siena, Pisa, Volterra, San Gimignano, etc. Also given its proximity to Florence we enjoyed that lovely city on 3 occassions. Rather than drive in Florence we drove to a nearby town and took a (frequent) bus into Florence. Very easy.
The owner of LeTori is a wonderful host, as his family. There is a welcome buffet each Saturday night, as well as cooking classes, and a meal during the week. There is a nearby (walkable) winery and for 3 euro we were well taken care of. It is a beautiful setting and great for walking, or bike riding too.

I think Florence is a place you want to stay a few nights - really nice in evening with folks strolling about - bridges over Arno nice lit up and then spend 5 nights in a place like west47 talks of - those AgroTurismo places may have bikes for short putzes though Tuscany is of course a bit hilly.

I would centrally locate yourself so you're not moving around all the time. If you are looking for someone to do personal wine tours check out Angela of Tuscany Tour Time.