Formentera is an island quite close to Ibiza, and popular for day-trippers. Here's an outline for an itinerary to get a taste of Formentera in one day.
First, there are a couple different ferry companies that go between Ibiza and Formentera. They're comparable in cost and all advertise a 30-minute "fast ferry" ride. Our trip time was more like 45-50 minutes! I can't imagine how long the 1-hour "slow ferry" would take...
Our first stop was Ses Illetes, a narrow spit of land going into the sea. I just couldn't get over how clear and blue the waters were here!