The climb and long descent West to East across the bottom of the island from Propriano via the Col de Bavella is a nice ride.
The coastal road between Calvi and Ajaccio is scenic (e.g. Calanques de Piana) and may be a good choice if you want to avoid riding in the centre of the island when there is the potential for thunderstorms and mountain weather. Also on the western side, ride inland from Porto to the Col de Vergio (lunch at hotel at the top of the col) and return the way you came back to Porto (when I did it, I descended inland to the station at (I think) Francardo and caught a train to Bastia, but using the railway probably would not suit a day ride, and I don’t know if they still take bikes).
Whatever you do, I would recommend a trip on the railway from/to Ajaccio (whether you stop/start at Calvi or Bastia), since the scenery is stunning and it’s a great way to see it.
