Florence

What exact date, and what sorts of things are you interested in?

We will be arriving in Florence Sunday, Sept.1 . We will be coming from spending the day in Milan. We leave Florence Monday, Sept.2 at noon on a biking tour.

Pretty much all of the museums will be closed by the time you get in and get settled, although Palazzo Vecchio is open quite late into the evenings on Sept. Sundays.

http://museicivicifiorentini.comune.fi.it/en/palazzovecchio/informazioni.html

Alternately, if it's a nice night you could trek up to Piazzale Michelangelo and watch the sunset/lights twinkle on around Florence. Google up some night pix, like this:

https://www.photojpl.com/florence-by-night-hd-panorama-from-piazzale-michelangelo/-/1Ccru5ZXsd/

There are a couple of open-air cafes up there for a beverage. if you hustle, old San Miniato church - just above that piazza - is open until 7:30 on summer Sundays (according to their website) so you might be able to sneak a short peek before she closes.

https://www.sanminiatoalmonte.it

Use for background about the church only:
https://www.visitflorence.com/florence-churches/san-miniato.html

All of the State museums, like the Uffizi, are closed on Mondays so just I'd grab your guidebook and do a self-guided walkabout. Most of them will include routes for one or more of those. I might suggest the Duomo complex but I wouldn't stand in line for the cathedral itself as you don't have that sort of time, and the exterior is more impressive anyway. The wonderful baptistry is also showing as closed Sept. 2nd on the website. The museum, belltower and crypt will be open, however. Order tickets in advance:

https://www.museumflorence.com

Some of the churches will be open but without knowing what your interests are....

Question? Is the bike tour departing from Florence? I'm just curious as you would be there for such a short time that I would scrap it entirely unless you have to be there for that reason.

Either way, if you have to meet the tour departure location in Florence by noon OR catch a train onward to a different meet-up location, your A.M. time is going to be short. You'll need to build in time to return to your hotel to collect your luggage and then get to wherever you need to be by 12:00.

Since your museum options are rather limited on a Sunday evening and Monday morning, I would use your 18 hours in Florence to relax between Milan and the start of your bike tour. On Sunday evening have a nice dinner and a casual after-dinner stroll around town, maybe buying a gelato along the way. On Monday morning you may have time for a secondary museum and more exploration of central Florence, but not much else. Piazzale Michelangelo is an option, or you could just limit your exploration to the historic center and Oltrarno area. Monday openings for secondary museums can vary from week-to-week, so you'd need to check the schedules for those of interest to you. In addition to those mentioned above, the Bargello and the San Marco museums are nice and may be open.

San Marco (which I enjoyed!) is open on the 1st Monday of the month; the Bargello (also very good) is unfortunately closed.

http://www.polomusealetoscana.beniculturali.it/index.php?it/190/museo-di-san-marco-firenze
https://www.visitflorence.com/florence-museums/san-marco.html

http://www.bargellomusei.beniculturali.it/musei/1/bargello/

Hello from one Iowa resident to another!
Would definitely travel up to Piazzale Michaelangelo for dinner and to see Florence at night. What time you have the next day I would just do a walking tour around the Duomo area and down to the Ponte Vecchio and the Orno River.

I can't think of any place worth eating dinner at Piazzale Michelangelo, so eat dinner somewhere else either before or after an evening visit. Personally I prefer the daytime to the nighttime views from the Piazzale, but both are nice for different reasons.