Hi, I was in Florence this spring and absolutely loved it. Some museums are closed on Monday but Sunday is good. Michelangelo's Galleria dell'Accademia and the Uffizi Gallery are both open on Sunday. They are both just breathtaking. Here is a link. I highly recommend buying reservations for both museums ahead of time which will save you a very long time waiting in lines to enter. Check out The Rick Steve's book on Italy, it's really helpful.
Have a blast!