Islanda
At the far western end of the peninsula on which Reykjavík sits there is a small island. This island, called Grótta, is connected to the mainland on low tides and open to the public most of the year (closed May 1 thru July 15). Just make sure you don't get stuck on the island when the tide comes in!
Listen to the audio guide for Grótta Free on WIP — starts by itself when you arrive, even with the screen off