Direct UBB (www.ubb-online.com) trains from Stralsund (and Greifswald) stop at coastal resorts before terminating in Świnoujście (Swinemünde in German), just over the Polish border. Peenemünde is on a branch line; change in Züssow (from Stralsund, two hours). A day ticket costs €16.50.
Ferries run between Peenemünde and Rügen Island.
Look for Usedom Rad (www.usedomrad.de) stations all over the island and Greifswald. You can rent bikes from machines for €9 per day.