Depending on your health, the climb to the top of Florence cathedral's Dome is great and the views wonderful. Can get quite sharp cold in winter, so be prepared to wrap up. Like others have said, big queues to see 'David' and the Uffizi. The big red tour bus is a reasonable way to see Florence from all around the edges and the hills on the far side of the river. Personally, I found Florence fine but was soon looking for more - I'd say a couple of days max. Very expensive shops if that's your thing.
As other s have said, the glory of Pisa is the 'Miracoli' area around the cathedral, campanile and baptistry. As a boring historian I loved this area, like buildersmate says ignore the tat and the Japanese tourists posing.....
Pisa also has a museum of roman ships found recently; a botanic garden, tiny and pretty in an unkempt way, many junkies who do not seem to bother you and some ridiculously overpriced restaurants.
The same fast railway line that connects Pisa and Florence also stops at Bologna, which I believe is worth a squint, and eventually Venice. Last time I went I flew into Pisa and out from Venice, so didn't need to retrace my steps.