The Reef Bay Trail--It's downhill all the way, and can be slippery, so don't wear flip-flops.  Great overgrown plantation ruins, old walls defining property boundaries and second growth forest, including bay trees (bay rum) and a monkey-no-climb tree (spikes covering the trunk.)  The side trip to view the Taino petroglyphs is worth the minimal effort.  The remains of the sugar mill at the end of the trail deserve your thorough exploration.  If you go in August, you can harvest Genips and Tamarind from the trees near the sugar mill.

The National Park Service offers a guided hike with lots of interesting facts and history.  A boat picks you up at the end of the hike for the trip back to Cruz Bay (the boat trip costs $10-15/person.)  Before the hike, have breakfast at the Texas Cafe, then catch the taxi near the ferry dock to the trailhead where the guide meets you.

If possible, read "Night of the Silent Drums," a history of the Danish sugar plantations and slave revolt of St John before you make the hike.  As you hike, let the surroundings, that story and your imagination take you back in history.  You will never forget it.