If you want to get a panoramic view of Florence, head over to Piazza Michelangelo across the River Arno. It is a bit of a walk from the town centre, and then a climb up to the Piazza. The square is dedicated to Michelangelo, and there is a replica of David in the middle of the Piazza. If you don't feel energetic enough to make the climb, you can also take a taxi up.
The views of Florence from the Piazza are spectacular - and these are probably the views printed on millions of post cards that you buy from the souvenir stands. Go around sunset for the most beautiful colours.