Yeah ..in my opinion 7PM is too early -Florence is wonderful at night. Perhaps there is a way to arrange a cab back to the Villa? Either way, if during the day ABD offers the opportunity to climb the Duomo -I would suggest doing that. I have slight asthma and I didn't think it was bad at all -when you reach the top you forget the effort you put into it. The views are amazing and knowing that you are walking inside Brunelleschi's creation was inspiring ...read up on him a little, the back story is fascinating. No matter what you do you will have a wonderful time!
Great advice. This trip is right around the corner. Will have to read up on Brunelleschi. So much to do, but now's the time!