Arno River area in August 22-25 for 3 nights

We stayed in the Sant'Ambrogio area, in an airbnb apartment on a pedestrian-only street called Borgo la Croce near Piazza Beccaria (where the little electric buses start and end their routes). Very handy. Still within the old medieval walls of Florence. Exactly a 15 minute walk to the Cathedral and near the Sant'Ambrogio market and other grocery stores large and small. Two minute walk to what I insist is the best gelato place in Florence.

Several people we took classes with who stayed in the Oltrarno complained of the heat, mosquitoes, and night-time noise in August and September, but I figure there are many Oltrarno apartments without these problems. No matter where you stay, make sure you get real air-conditioning so that you can shut your windows at night.

If you stay in the Oltrarno, I would choose a location for which "your" daily bridge is not the Ponte Vecchio. Much more pleasant to SEE the Ponte Vecchio from upstream or downstream than to be ON the usually crowded Ponte Vecchio, although of course you will want to walk across it at least once, ideally early in the morning or late in the evening. If you stay near Santo Spirito (which we did on another trip, but in December), it's a 15 minute walk to the Cathedral.

I just stayed a month in Oltrano only a few streets from Santo Spirito and it was a quiet location. Streets with no bus traffic (and thus no heavy auto traffic) are generally quiet. It was a 15 minute walk to Cappella Medici (near Duomo) at a steady pace. Much of the Santa Maria Novella area is very busy and imo not quiet. I've also stayed in the Sant'Ambrogio area and it was generally quiet, although something right on a busy square or street wouldn't be. Ask again as you narrow your choices and people can comment on the exact areas you're considering. One more thought, apartments on higher floors tend to be quieter but can come with a lot of stairs.