Florence- Duomo-?'s

What I’m about to say might be heresy, but I would skip going inside the main church (the lines are long) and would instead prioritize enjoying the magnificent exterior and then visiting the Museo dell’Opera del Duomo in the piazza which houses much of the art that was originally in the Duomo and where you’ll have an opportunity to view the art up close.

By all means climb the dome — I can’t advise on tickets because I’ve never done it. I think the most beautiful views of Florence include the dome so I chose instead to view it from Giardini Bardini one year and from Piazzale Michelangelo/San Miniato al Monte another year.

Just returned from our Florence to Rome trip.
I strongly recommend booking a timed pass to climb Brunellesci's dome with the Brunelleschi's Pass that is valid for other sites for three days.
Without this timed climbing pass, I wouldn't have seen the majesty of the city for the first time.
https://tickets.duomo.firenze.it/en/store#/en/buy?skugroup_id=3006

It’s been a few years since I was there as part of the Venice/Florence/Rome tour, but I do recall the long line-up. Myself and another tour mate decided to climb the dome in our free time, and when we saw the line-up we asked a person in the line how long they had been there and they said about half an hour or so ( they were close to the front), but also told us we couldn’t buy tickets there, they had to be bought at a location down the street. So we went and bought tickets, and waited in line about 45 minutes to get in. But, we had some nice conversations with other travellers. Unfortunately, there were some people standing in line without realizing they couldn’t buy tickets when they got to the front. The climb up can get challenging towards the top as it gets lower and more narrow, and people come down the same way so you have to squeeze up against the wall. It can also get quit stuffy if it’s a warm day. I had caught a cold, and was quite stuffed up, so had to stop and catch my breath a couple times. I enjoyed the experience, though as you can look down at the church interior at one point, and also up at the dome interior. The views from the top are spectacular. But, I would definitely buy a timed ticket if you can, as I think the whole process took us almost three hours in the end.

I haven’t climbed the inside of the Dome, as I would feel a bit panicky in there I think.
However, I’ve climbed Giotto’s bell tower a number of times.
The steps are fairly wide, no spiralling, and there are wide landings to stop on, on the way up.
Just don’t sit under a bell when they ring!
You then get to see the Duomo up close from the top, and 360 degree view of the city.
Not as crowded up there.
I think you do need advance tickets nowadays.
The Museo dell’Opera del Duomo is wonderful.

I am currently standing in line for the crypt right now. I was just hunting the forums because I had no idea it would be this crowded even though I bought tickets months ago! They make you leave even small backpacks in the luggage storage, and then you have to stand in line in the hot sun without water. My 12 year-old had to use the bathroom, so I ran around with her hunting for a place; we tried to go inside the museum, but realized after going through security that they were going to scan our ticket so we couldn’t go back in. We finally used the museum cafeteria restroom, and got glared at. My other two kids had been standing in line and got to the front pretty quickly, but then they had to get out of line because we weren’t back yet. Hopefully the crypt is worth it, because we had to stand in line all over again. We are not even going to have time to get inside the church. If I’d realized it would be this bad even with advance tickets, I would’ve tried to get here at 10 AM when it opened. I’m hoping we have time to climb the tower and see the Baptistery, before our sunset timed tickets to climb the dome. Pretty frustrating for the amount I spent, and half my kids were child price!

Update: it turned out to be not that bad, although I would definitely recommend going first thing in the morning! You can indeed see the interior of the church by entering through the crypt entrance. I am glad we didn’t wait in line for the front of the cathedral! It’s just not very clear how it all works.