Italy in Late October - Itinerary Recommendations

Hi Ryan,
We just visited the AC this past March, using the 'shoulder season' strategy to minimize crowds. We deliberately based in Minori as part of that thinking. Things went well. If interested, we can provide the contact for our rental, a fantastic-value apartment with that rare thing among moderately-priced AC rentals---a lift! Can also recco a certain splurge on Ischia.
We've visited Santorini shoulder season as well and can attest that it'd be a great choice. Still some crowds mind you. But one could conceivably pair that isle with another less touristy one called 'Folegandros'. That isle really hits the sweet spot of shoulder season but still comfortable on all levels. Can recco places to stay there too.
Spain too can be great in the Fall.
Good Luck.
I am done. the end

I know that the attractions to hop somewhere else is strong once you have already crossed the Atlantic, but I would still encourage you to stick to Italy because ten days really does fly by...and there are just so many attractive options in Italy!
Sicily is indeed by far the easiest--but it also a great choice for early fall! Once you are there I cannot imagine why you would waste time going anywhere else. I have yet to make it to Sicily--there are many trip reports here. I typically start with a official tourism website before delving into guidebooks: https://www.visitsicily.info/en/
As a hiker who loves coastal areas like the Amalfi Caost and Cinque Terre, I've had Zingaro nature reserve on my radar. With 12 days, you'd certainly have time for that. Also look for Cicerone guidebooks, they specialize in hiking. Good luck!

Another thought: If you find the flight logistics painful (you might be looking at an expensive flight to Sicily) or if Sicily just does not float your boat, breaking up the trip in Spain might be economical, and it's a good time to go there as well--avoiding the brutal heat of summer. Some flights are seasonal, but look at all of your options to Catania as well as Palermo.

Another vote for Sicily! With only 10-14 days, why spend time traveling to somewhere else? Are you planning to spend your extra days before the wedding or after? The weather is great in October, but less so in November; it's cooler with a bit more rain. Still better that the north, though. We've spent 6 weeks over the past 2 years there and I'm up for at least 6 more. Sicily's got everything -- cities, countryside, ruins, art, churches and cathedrals, amazing food, museums, small towns. I'd suggest you take a look at all the Sicily trip reports here, get the book and look at the tourism websites. It's nearly impossible to do both the east and west sides of the island any justice in a short trip. While we liked the eastern side (Siracusa, Etna, Catania, etc and daytrips.) I especially enjoyed western Sicily and would recommend Palermo and Trapani with day trips from each. It's easy to get to Cefalu and Scopello from Palermo. And from Trapani, San Vito Lo Capo, salt flats, Segesta, Favignana, Selinunte, Erice, Marsala, Mazara del Vallo and other interesting spots are a short train, bus or car ride away. Googling "western Sicily" will bring up a bunch of travel blogs. We found alongdustyroads.com to be very helpful.