What to do in Florence for the day on a third Sunday

Many of the well-known sites are open on third Sundays unless they're closed on EVERY Sunday. As far as making suggestions, what sorts of things are you interested in?

It's our first day in Florence, and since we'll be back later, I think I am looking for an introduction to the city and maybe some of the more accessible sites that are open on Sunday: an overview of the city and one or two museums?

One of Rick's self-guided walks ? Gelato ?

As Laura has suggested, a self-guided walkabout in Rick's or other travel guide is a good idea. Rick also has some free audio tours you can download: the "Renaissance Walk" might be fun?

Link to the audio tours:
https://www.ricksteves.com/watch-read-listen/audio/audio-tours/italy

And here's the accompanying map for that tour:
https://d3dqioy2sca31t.cloudfront.net/Projects/cms/production/000/004/456/original/f854d0e8c9b4a3aded25efdb2bce1e92/renwalk.pdf

How long will you be in Florence later in the week? There are museums - particularly the Uffizi - you're going to want to pre-reserved tickets for in advance to skip very long ticket lines.

A couple of great places you could visit other than Uffizi/Accademia on your Sunday:

San Miniato al Monte and Piazzale Michelangelo:
Highly recommended! Schedule your visit to arrive and tour the (very old and interesting) church before the 5:30 service with Gregorian chant, and then attend that service for the experience. Views from the church and piazza just below it are breathtaking:

https://www.visitflorence.com/florence-churches/san-miniato.html

https://www.visitflorence.com/florence-monuments/piazzale-michelangelo.html

Bargello:
https://www.visitflorence.com/florence-museums/bargello.html

Santa Maria Novella:
https://www.visitflorence.com/florence-churches/santa-maria-novella.html

Santa Croce:
https://www.visitflorence.com/florence-churches/santa-croce.html

Great! Thank you for the suggestions.