The island is just a short trip from Cannes by boat.
Well worth a visit. The island has well maintained paths to explore, beautiful little beaches and crystal clear water to swim in. There are also some lovely restaurants and also the fort where the man in the Iron mask was held.
As other reviewers have said comfortable footware is a must for exploring the island, also if you are planning on swimming the water is quite rocky so bring some shoes for getting in and out of the water. There are lots of spots to picnic although there are also some restaurants near to the dock/fort (walk along the bottom road from the dock).
The island can get very hot in the summer too so make sure to bring water, sun cream and a hat!!