In my opinion a relaxed Sunday in Florence is hard to beat. That lazy day when you leave the house to have coffee and breakfast out and then just go from there. A walk in the center or along the river, lunch in the sun (hopefully!) and then nothing is better than a flea- or antique market! In Florence there are markets almost every Sunday and you just have to go to the right zone and square to find them.
One of the most popular markets is in Piazza Santo Spirito the second Sunday of every month. Here you can find vintage clothing and accessories but also antique furniture and many other items. The bars and restaurants around the square make it all more of a social event than most other markets. You can have a break at one of the bars or have lunch and then continue to browse the stands until late in the evening.