Hot Springs - Florence/Tuscany Area

sink,

For hot springs, you might have a look at Bagno Vignoni which is in the Val d'Orcia area of Tuscany. As I recall, there is at least one hotel there and also a very nice restaurant. If you'd rather not stay right at the hot springs, there's also a very nice hotel at within a short walk from the hot springs.

Can't help with the Vespa rentals.

Tuscany is the region with most spas. There is spa galore.
Do a search on Google. There are several websites that will list many. Enter the key words: TERME TOSCANA.
My personal experience is limited to Bagno Vignoni, Terme di Saturnia and a couple of less famous ones.
Florence has several scooter rental places. However going to the Siena area with a Vespa is a long way because you can't use the freeway. I presume you have experience riding motorcycles, because Italy is not the place to start.
If you do the Chianti road is a good ride. It's a narrow highway, therefore you need to be careful.

Thanks, guys.