With its steep terrain and long roads, Orcas Island [1] is definitely the toughest of the islands to get around on bike. However, it can be pretty fun if you're up for the challenge.
In Eastsound you can pick up a rental steed from Wildlife Cycles (350 N Beach Rd., 360/376-4708, www.wildlifecycles.com [2], 10 a.m.–6 p.m. Mon.–Fri., 10:15 a.m.–4:45 p.m. Sat.–Sun., winter hours vary so call ahead, $30 per day).
If you prefer to rent a bike at the ferry and take the long haul into Eastsound or other remote parts of the island, Dolphin Bay Bicycles (360/376-4157, www.rockisland.com/~dolphin [3]) is right at the ferry landing and rents bikes for $30 per day.
Links:
[1] http://www.moon.com/destinations/washington/san-juan-islands/orcas-island
[2] http://www.wildlifecycles.com
[3] http://www.rockisland.com/~dolphin