Late September to early November is really our summer time, fog is gone & weather is glorious in California. What about SF or LA? Lots of museums, cool neighborhoods, fabulous food. I prefer SF for visitors....
SF is a perpetual delight & you don't need to rent a car. Smaller, easy to get around by Lyft or Uber than LA. Join the SF Fine Arts museums of SF. (Call them if you're over 65, don't recall but it was even cheaper & if you and a guest go to one exhibit it's basically the same price as an annual membership.) The Treasures of The Pharaohs is opening soon at the Legion of Honor which might be one of the finest locations of any museum, overlooking the Golden Gate Bridge. https://www.famsf.org/learn-engage/read-watch-listen/5-things-to-know-ancient-egypt-treasures-pharaohs. I'm not a modern art fan, but the MOMA is pretty fabulous. The Asian Museum is also delightful. Please don't stay very far into SOMA, South of Market Street, it a bit dodgy. We've stayed at the Hyatt Regency close to the ferry docks. Great location. https://www.hyatt.com/hyatt-regency/en-US/sfors-hyatt-regency-san-francisco. I love the walk from Crisey Fields to under the Golden Gate Bridge. If you're feeling up to it, you can also rent bikes near there (don't remember, maybe on Market Street? Google it) & ride to Sausalito over the GGB, take the ferry back! Sausalito is a cool little expensive town, amazing water views & I can look up the cafe I like if you're interested. Don't miss the last ferry back or it's all uphill to the GGB! Anything heading 'up the hill' from the Bank of America Building to Top of the Mark has great views and Victorian architecture. Of course there's always the cable cars to make your way back.
In LA you can plan ahead to visit what I call the OLD Getty in Malibu, (read up on how to get there & be dropped off if you're not renting a car. & you need a timed ticket.) I would suggest the La Brea Tar pits but it just closed for 2 years of renovation. There is often hellish traffic, so timing on where and when you drive is essential. The County Museum is amazing, yes there are old neighborhoods but I don't recall, someone else will have ideas. Hope you enjoy your visit!