Pigeon Island National Park is at the North end of St. Lucia. It was declared a national park in the 70s and joined to the mainland by a manmade causeway. It is about 45 acres and is open from 9am to 5pm for USD$4 per person.
There are quite a few ruins, as this was a strategic point during the 14+ times the island changed hands between the French and the British and even before that. From the top of the old fort, you can see Rodney Bay to the south, and on a clear day, the island of Martinique is also visible to the north.
It is quite a hike up to the fort, but the path is wide, and we actually saw a family with a stroller, slowly inching their way up.