Your itinerary doesn't look possible in 5 days. Here's a list of minimum time needed to enjoy these places:
Florence 1 day
San Gimignano 1 day
Pisa + Lucca 1 day
Siena 1 day
Volterra 1 day
A 5 Terre trip in 1 day is possible, but you would be spending something like 4 hours just traveling to and from...and then would barely have time to walk around, have lunch and head back.
I would sugest cuting out Volterra if you really can't add more days to your plans.