Need advice for 10 days in one location??

What about Lucca? It is a similar sized town to Siena, but very different. There are some fairly easy daytrips if you do find yourself a bit bored. It has the lovely walls to walk or bike around.

Sulmona might fit the bill and it's very close to Rome and easy to reach from that city.

Lucca is also a good idea.

I don't think the Amalfi Coast would give you a feeling of living like a local in September. Neither would a town on Lake Como; these are all massively crowded in season and September is very much high season.

What about a small city like Ferrara?

Ferrara is great!

Are you into biking? The Mantova area is a popular biking area.

Or, there’s a great network of bike paths in the Puglia region. It would certainly give you the beautiful water settings, too. Even though I move locations very often, the Puglia region oozes relaxation. Or at least it’s that environment in May. ; )

It’s your second time in Italy and you didn’t mention Florence as a place you’ve been.
The answer to your question is obvious: Florence.
You can easily be busy for 4 days in Florence, but in addition Florence is Tuscany’s capital and transportation hub, so from Florence you can take a variety of day trips to other Tuscany locations, for example:
Lucca and Pisa (train)
Arezzo and Cortona (train)
Pistoia (train)
San Gimignano and Monteriggioni (bus)
Volterra (via train and bus via Pontedera)
Chianti hills (private tour)
and of course Siena, if you care to return.

Florence is also well connected to places outside Tuscany, such as Bologna, and even places in Umbria (Perugia, Assisi).

We based in Sorrento for two weeks in 2023 and loved it. Here's my Trip Report if you think it may be useful. https://community.ricksteves.com/travel-forum/trip-reports/trip-report-2-weeks-of-day-trips-from-sorrento

As others have suggested Lucca could be a good choice but also there is Arezzo which is sort of the step-sister to Siena would be interesting and more authentically "Italian" than some other places more on the tourist map. There is no "living like locals" on the AC since there is only a crushing tourist season and then shut down.

Virtually any seaside destination in Italy will be busy in the summer since people take to the seaside for days or weeks at a time. The Ligurian coast north of CT has a bunch of interesting towns - Chiavari, Sestri Levante, Moneglia - and you'll be among tourists but I bet they will primarily be Italian tourists.

If you like the coast Salerno might be an interesting choice. I think of it because it is part of Campagna and close to Naples it has the most "land based" menus of any seaside place I've been in Italy. Anything with a tomato sauce is crazy good. There are many good seafood restaurants but also many land or "both" menus in play. Salerno is surrounded by interesting things to see - including the AC - is you want to add some day trips to your "local" experiences.

For background I spent a month in Lucca in 2024 and here are some thoughts:
Good:
- Lucca (the old town) has way more good restaurants than it should for the size. I call Lucca 'cultivated' Italian life because it collects the best of the surrounding area. (There are no muffler shops in the old city.)
- It does have all the things you want - bakeries, fruit and veg stores, weekly markets.
- There are multiple free or cheap music events every night in September.
Bad:
- It will be hot. Traditionally near the end of September the weather breaks and evenings start to be cooler. Living like a local will mean getting out early and retiring for siesta for the hottest part of the day and re-emerging at night.
- It is touristy so there are no deals to be had and you will pay retail for your lodging.
- The center is clogged with tourists during the week and while not like Siena you will hear more English (American and British) and German than you may want.

Be sure you get a place with a kitchen, AC is probably required by anyone mortal anywhere in Italy in September, how much Italian you speak may determine how much like a local you actually live. The further you get from touristed areas the amount of English spoken drops off dramatically.

Whatever you decide - have a great trip,
=Tod

For a lake town, consider Stresa on Lake Maggiore. Charming little town. Lots of restaurants. Beachy area for swimming. Islands to explore, plus lots of towns along the lake. 45 minutes to Milan, or take the train into the Swiss alps. So many options. For just chill by the lake.

This is all so helpful!! Thank you!! Lucca has come up several times. It sounds like our speed. I hadn't thought about it being so hot that time of year. I’m from Phoenix so maybe it wouldn’t seem awful.
We had considered spending the bulk of our time in CT also but would it be too busy to hike at that time of year?
We haven't been to Florence and everyone that visits seems to fall in love with it. I wondered if it’s too big for a relaxing trip? We are wanting to make sure some of our time is spent there so now focusing on locations closer to Florence.