Cinque Terre & Sienna day trips or more time in CT?

Since you are headed to a seaside location in Sorrento, I would skip the Cinque Terre day trip. It is a long way to go for a few hours. Three nights in Venezia is a good way to start your trip, a chance to get over jetlag (or at least get on top of it) and two full days to see highlights of this unique city. I also like 4 nights in Firenze and a day trip to Sienna is easy peasy. I personally feel San Gimignano is not worth the effort, so if you are more drawn to things in Firenze (endless possibilities!) don't force yourselves to go there. Besides combining it with Siena would make for a long day. Siena has plenty to hold your attention for a day and it is a more real city than San Gimignano.

BTW, 2 nights in the CInque Terre might be tempting but it dilutes your other locations and really one day there is not worth all the travel time you will put in coming-and-going.

I think we often minimize how tiring the travel days and day trips can be. You don't want to spend every day on a train or bus going somewhere other than where you are staying! It is exhausting!

I would do either Sienna or San Gimignano, not both, especially on the same day.

And I would also skip the CT. Laurel is right about travel time and day trips. I know some people love the CT, but it's awfully crowded, with lots of touristy things. (Not that that's necessarily bad, but if you're only there for a few hours it's not all that interesting or educational.)

I think at least three nights in Venice, with plenty of time to get off the beaten track. And Florence is packed with interesting things to do, especially if you are art lovers.

What about the CT attracts you? If you really want to go, spend more than a few hours. Or, if you do go, spend the time on the ferry that skirts the villages. The scenery is breathtaking, then you can take the train back to Florence.