In Siena try eating at Osteria CeCi. A hole in the wall place, sort of subterranean. Watch your head as you enter if you are taller than around 5'9". Not much English spoken here but some terrific pasta.
In the Oltrano area in Florence, look for www.trattorialacasalinga.it I assume it is still there. Haven't been to Florence in 7 years. (Must get back)
As for sights, there are some art restoration places in the Oltrano area that are really fascinating. I loved the sculpture at the Pitti Palace.
In Venice, take in the opera house and make sure to go to the Guggenheim museum. Eat at Vini da Gigio or Trattoria Alla Scalla, which is a seafood place not too far from the clock tower.