The Duomo is a must see, an incredible building, and the interior of the dome is mind blowing.
-1 for the Uffizi (IMHO).
+1 (this could get confusing) there’s just to much to see for a days visit, you walk around looking at incredible painting after incredible painting, and it just sort of washes over you. I would suggest if you seriously like your art to spend 2 days at least in there. There are many, many other museums, art galleries and churches with the most incredible stuff in, it seems a shame to ‘waste’ a day in the Uffizi.
Boboli Gardens +1
Just walking around Florence, stopping and looking in churches is a great thing to do, we went for three days, and alot of that time just walked about, taking it all in, stopping for a coffee every now and again.
Oh yeah. Vivoli ice cream. The best ice cream ever. you have to go.