The Cova de s’Infern (Cave of Hell!) is, by a long way, one of the most incredible geographical accidents in the Cap del Creus. Visiting it is an amazing experience. It's basically a rock that, thanks to water erosion, has gradually been shaped into a kind of cave with a natural swimming-pool inside it. Take a dip there and you will practically arrive at nirvana. The name, incidentally, comes from the red colour you can see in the water as the sun is rising. To get to it, you need to walk about 10 minutes from the lighthouse.
Visiting sea caves on the Costa Brava is one of the most spectacular outings that you can do in the area. You'll need a kayak or a boat, and your mobile to capture the sights, but it's definitely worth the effort. Really worth it.