I rented my car from Europcar and they do allow the short ferry crossing from Corsica to Sardinia. The island isn’t that big so it would be easy to visit all the sites in the time you have. Some restaurants and hotels might be closed but there are many that will still be open.
I was only on Corsica for 4 days, 3 nights. One great day trip is to drive anti clockwise around Cap Corse on the coastal roads The scenery is great. If you’re adventurous, drive to Barcaggio at the northern tip of the island. Stop into the Pizzeria Marine in Marine d‘Albo for a quick bite and nearby nice views from the beach. The black sand beaches of Ogliastro and Nonzo, nearby, are spectacular. Then drive to Saint Florent and stroll around the port. From there return to Bastia and enjoy moules et frites at Café Albert 1er on the Place Saint Nicolas.
I next headed to the west coast taking the D84 to Marine de Porto which is a marine sanctuary and more beautiful views and a great beach. I stayed at Logis Hotel la Lonca which offered free parking. For a light lunch, try La Cigale across from the Spar supermarket - they have a sandwich called “The Big” which is a hamburger on a baguette 👍. For a wonderful dinner with local Corsican cuisine, choose Restaurante Le Moulin.
The Calanques of Piana are a few km south of Porto on the way to Ajaccio where I hope the market will be taking place on Place Foch. I didn’t stay overnight here but really enjoyed the strolling the city of Napoleon. Had an enjoyable lunch on the terrace of Produits Corses a Casetta on Avenue du Premiere Consul. Next stop was one of my favorites - Bonafacio! A spectacular marina/harbor, seafood restaurants, a castle, boat tours of the chalky white cliffs. I stayed at the quirky Hotel du Centre Nautique right on the quay. (free parking) There are dozens of restaurants on the quay from which to choose, mainly seafood. Some good Italian restaurants in the area around the Bastion.
If you want to go to Sardinia, you can take the ferry to Santa Teresa. If you don’t take your car, visiting the town would be a short day trip. If you take the car, it’s about a 2 hour drive to Alghero. Longer if you visit Castelsardo or Porto Torres. I visited the former, not the latter. In Alghero I stayed at the Hotel Domomea with only on-street, but easy to find, parking. I’d suggest taking the boat trip to the Grotta di Nettuno from the harbor. Ristorante Mirador on Via Manno near the Bastion had good seafood. For the best pizza in Sardinia, OK Pizza Evolution is about a 5 minute walk from the hotel on Via Sebastiono Satta. Make sure to have Grappa when you’ve finished your pie!
Back to Bonafacio for the trip to Bastia. I drove up the east coast visiting Porto Vecchio then driving inland from Aleria to Corte. If you enjoy challenging roads, leave the T50 and detour on the D14 which eventually returns to the T50. And then to Corte and lastly back to Bastia! Whew!
The roads are twisting and turning so plot your travel on Via Michelin to get an estimate of the time required to get from A to B. Looking at a map is very deceiving. Also be careful of livestock on the roads (sleeping pigs look like large rocks), wild animals and hunters. If you have a smart phone that uses a SIM card, just buy a local sim when you reach France, either at CDG or when you arrive in Bastia. I would head to a cell phone store once I’m in Bastia because you’ll get better service and more options. If you don’t have a smart phone that’s unlocked, I would buy one from Amazon or EBay before you travel,
Get to Corsica as early as is practical because sunlight is at a premium in November. If you want more info on Sardinia, you can send me a private message.