Portovenere/ Cinque Terra/Ravenna? or Assisi/San Marino/Ravenna?

Where in Tuscany are you staying that week? And do you have a car?

You might be able to visit Assisi during that week. It's about 1.5 hours drive from Siena and 2 hrs from Florence. Same with the Cinque Terre. Depending on where you are, if you have a car, you might be there in 2 hours (it's 2 hrs from Florence by car).

Ravenna is (somewhat) along the way between Tuscany and Venice. Other options could be:
Bologna, Ferrara, Padua and even Verona (slight detour).

San Marino is a pain to reach, with or without a car. By car you could cut through the mountains, but the drive from Sarsina to San Marino will upset the strongest stomachs.

Grazie, Roberto.