Definitely bring layers (sweater/light jacket). October can be chilly, particularly in the evenings.

You’ve mentioned a lot of chain restaurants. I don’t really go to them, but I get the appeal for someone who’s never had those.

DC has tons of good local restaurants. Definitely think about venturing out from the chains. Rather than listing random restaurants, I’d just suggest browsing the Michelin guide/Google maps/Washingtonian magazine top 100 for ideas. You’ll find plenty of non chain options.