No need to additionally emphasize how worthwhile a visit to this WHS is, the rating and the previous reviews are unambiguous. So just a few short comments and recommendations:

I visited Florence several times, both as a tourist and for buisiness reasons, most recently for four days in November 2017. And that is also the best time for a visit in my opinion: still pleasant weather with good chance for sunshine and less tourists, at least in comparison to the high season (from Easter to All Saints' Day). However, there are still long queues at the most popular sights. For the Cupola of the Duomo, I would suggest to pre-book tickets and a time slot, there is only a limited amount of day tickets available. Pre-booking is also recommended for the Uffizi, unless you just want to see the highlights, then a late afternoon visit is the best choice. If your time is limited and you want to visit as much as possible, the FirenceCard might be an option, it includes priority access to all museums and sights (with the exception of the Cupola). But it is quite expensive: 72 euros for 72 hours. So rather suitable to skip the queues, not to save money.

My specific highlights:

• the Biblioteca Medicea Laurenziana and the Michelangelo stairs at San Lorenzo
• the Cappella de Pazzi at Santa Croce
• the Fra Angelico frescoes at the San Marco convent
• the bars and restaurants around the Piazza di Santo Spirito also popular with locals
• the Farmacia di Santa Maria Novella (perfume, cosmetics, herbals and much more is offered in historical rooms; perfect for a souvenir or gift, but check the price list bevorehand, it's pricy; but just looking around is also worth it)
• the view from San Miniato (photo)

One of the best things to do in Florence is to stroll through the city, looking for the remote corners (yes, there are some), enjoying the atmosphere and taking a break for a coffee or a gelato. Having said that, I must also admit that a bit of planning is helpful. At least, you should check the current opening hours and closed days, they differ between the various sites. As an example: the four parts of the San Lorenzo Complex (Medici Chapel, Basilica, Cloister, Biblioteca) have different opening hours, three entrances and two ticket desks.