The Río Istián is the slow, meandering channel that drains the swampy isthmus between the two volcanoes into Lake Nicaragua. A two- to three-hour kayak from the lakeside takes you through mangrove tunnels and lily-choked backwaters thick with birdlife — herons, kingfishers, caimans, the occasional troop of howler monkeys overhead — and gives you the most peaceful perspective on the island that exists.
Most guesthouses on the Maderas side rent kayaks and arrange guided trips; the river is calm enough to paddle solo if you know what you are doing. Go at sunrise if at all possible: the light, the bird activity, and the absence of wind are all at their best.