One of the things that should be done when in Florence is climbing the Duomo (and the tower bell additionally). Be ready to feel like you will suffer a heart attack – unless you are fit (something we are not). The 463 steps to the dome and the 414 of the bell tower will prove your physical endurance but the sights are just so worth it.
I also recommend you to be ready to queue or to arrive really early in the morning. We were lucky and didn’t wait too much for the tower bell (we arrived early) but we made the wrong decision trying to climb the dome after lunch… we waited in the line for two hours!!! Had we bought the Firenze card (no queues – win!) this wouldn’t have happened but since we had so little time we decided to visit the basics so the price of the card wasn’t worth it. Anyway, I can easily fall asleep on the most incredible places so I napped on a bench for a while – i like living on the edge – so it wasn’t that bad.