I went last month. Definitely book now for anything you want, we left it till we got there and couldn’t get tickets for some things. You don’t need a guide, go to the official website and just book a timed entry. I couldn’t compute the combination tickets, but they can work out cheaper.
We queued to get in the cathedral (no tickets for this) for one hour 40 minutes, and frankly it wasn’t worth it. There’s not much in there apart from the ceiling which is magnificent. Opted not to climb the Duomo, I don’t do heights and apparently the passages are very narrow, but we’d left it too late for my friend to get a ticket anyway.
The Boboli Gardens were beautiful, probably the best bit of the trip. Pitti Palace ok but crammed with art and not tremendously well explained. While you’re over that side of the river, it’s certainly worth climbing up to the Piazzala Michelangelo, a steep climb but fantastic views.
You need to book for the Uffizi and despite the timed tickets, it was very crowded. You can download a floorplan on your phone. Every other person seemed to be filming (not permitted) on their phone or taking selfies in front of the Botticelli Venus. There is some fabulous art there, but I did not enjoy the experience.
Some of the small museums are much better value, Dante’s house we were able to just turn up and walk in, and the Palazzo
Davanzati is a mediaeval nobleman’s house, really interesting and again you can just turn up.
We were unable to get tickets for the Accademia, but to be honest, we were a bit sick of art museums by then. Florence is very, very busy; walking down the pavement, you invariably have to step into the road or someone else does, to let you pass. I’m used to central London but found it quite overwhelming.
A great restaurant is Simbiosi on via de Ginori, there’s a pizza restaurant and a trattoria-type place side by side, great food and a very laid-back atmosphere that I think teens would enjoy. You can book tables online.
My advice would be book now for anything you really want to do, e.g. the Uffizi, climbing the Duomo or the bell-tower, we definitely left it too late. But I would also choose some of the less busy things that you don’t have to book for, San Marco museum is good apparently although we didn’t get there. I completely understand about wanting to make the most of it, but don’t pack too much in. Sitting with a coffee or a gelato and soaking up the atmosphere is also part of it in my view. I hope you have a great time, take comfy shoes!