Located on the southern tip of Corsica, the city of Bonifacio is the most southern French commune. On a clear day, you can see Sardinia from the town.
My suggestion
You should definitely take a tour of the city by sea, from where you'll see the citadel at the bottom, as well as several spectacular rock formations, such as "le gouvernail" or the famous "grain de sable".
The GR20 is a difficult trek that is known to walkers from all over the world. It crosses Corsica, from north to south, or the opposite, in 15 days of intensive walking. But seeing the sunset over the Aiguilles de Bavella, or the sheep on the mountain ridges, not to mention the unforgettable views of the sea - aren't these natural wonders worth the effort?