The road around L'Isle-aux-Coudres is the main experience of the island. The loop is roughly 23 km, mostly gentle, and keeps giving you new angles on the St. Lawrence, the Charlevoix shore, orchards, old chapels, and tidal flats.
You can bring a bike on the ferry or rent one near the island side. Allow more time than the distance suggests, because the best parts are the stops: a mill visit, cider tasting, river photos, and a long look from the western end when the light is good.