You can't not visit the Duomo when in Firenze. Climb to the top if you're fit enough (seriously steep climb) - the views are fantastic.

The cathedral is beautiful and the 8 euro fee to climb the 400+ stairs to see 2 internal views and one external view of Florence is worth it. There were very few tourists in December so we did not need to book in advance. The stair climb up the belltower or dome ARE NOT for those who are claustrophobic, have a large body size, fear heights, or have respiratory/cardiac problems. The same stairs are used on the way and down so you have to squish against the wall to pass and let pass. Half-way up you have to walk a narrow internal walkway that gave the couple in front of us some problems because she was queazy. That said, it is beautiful so I highly recommend. We didn't plan it but the sunset was phenomenal.

An amazing Cathedral with the magnificent Dome. You won't see this kind of architecture anywhere, so make most of it while there! Go into Cathedral, the entry is free. Climb up the Dome if you can, the fee is 8 Euros, but money well spent. The view over Florence right up towards the hills are stunning and you can take lots of great photos.Just one warning, there are 463 steps, at times it is a bit steep and in parts there are people going up and coming down at the same time. If it is hot too it can feel claustrophobic. I would not recommend it if you have respiratory or heart problems. You can also enjoy the Cathedral and the Duomo by sitting outside in one of the little restaurants or pizza places and soak up the great atmosphere over a pizza and a glass of wine followed by a yummy gelato!

It's simply beautiful! A must see if you visit Florence. Lots to see around the Duomo, and just sit and absorb the atmosphere around the Cathedral.