Everyone's experience is different, but while I was glad to see Florence I was glad to get out of it also! Far too crowded and massive queues for everything. But that's just me, I was part of the big crowds too I have to remember! One day or less is more than enough to see the main highlights, unless you want to queue for the Uffizi and all the other things you can queue for IMV,
I echo Lucca, a charming place, but a couple of days is enough. Siena is a lovely spot, but in the end I found a lot of the towns were very similar, same narrow streets, same architecture, similar churches, same food etc. Remember this is just MY experience but it is warts and all.
Then I got to Bologna..... what a mad place, I really liked it, a quite busy place, not sanitized too much for tourists, and is a total foodie heaven.
Moving on up, Padova (Padua) is delightful, a university town with plenty to see and do and great train connections. Direct train from there to Venice in less than half an hour and a great way to see Venice without paying through the nose for the overpriced accommodation there.
I could go on, but I won't. I did this over 12 days all by train. So easy in Italy. Flew into Pisa and out of Treviso.