The price of the 15-minute boat crossing (14 euros return) is a bit steep, but unless you have your own yacht or fancy swimming from Cannes you don't have much choice. Looks like all the boat companies collaborate to some extent to ensure that no-one could ever be left on the island, even in high summer when there are masses of people expecting to return to Cannes late in the afternoon. Perhaps the EU anti-cartel bureau (if there is such a thing) would care to investigate whether there is free and fair competition here.
Anyway, I digress. We walked around the whole island, and it didn't seem as much as 5 miles (as stated). It's flat (kinda obviously) and there are plenty of little stony beaches but no sand. Jellyfish were in evidence on the day we went there, so swimming wasn't an option. There are some great views to other smaller islands and back to the mainland, and much of the walk is in shade.
Best to take your own refreshments, although there are some food/drink outlets there (I suspect their charges reflect the fact they have a captive clientele). Also: no smoking on the island due to fire risk (they ought to announce that on the boat too).
As others have indicated, it probably gets unpleasantly crowded in high season, although many people can't be bothered to walk more than 500 meters from the landing stage...
Photos taken with iPhone 6s Plus.