We hiked the Reef Bay Trail in late July, without a guide. You don't really need one, as the plants/trees are marked with informative signs, as are the sugar plantation ruins. Obviously, if you hike without a guide you can also travel at your own pace.
Parking is limited at the top of the trail head; we got there about mid-morning and didn't have any trouble finding a spot.
As other reviewers have pointed out, the trail is downhill on the way to the beach. We saw lots of hermit crabs and large spiders/webs, in addition to all of the cool plants, trees, and ruins. The petroglyphs, which are about a 20 minute side hike (that's to and from) were also neat.
When you get to the bottom of the trail, that trail that leads to the swimming beach is to your right, prior to the plantation ruins (note: there are pit toilets located at the ruins). So you'll have another 10 minutes or so of walking (up and down) to get to the beach, but it's worth it. We packed our lunch and ate/swam in solitude. (The beach that is on the other side of the swimming beach is rocky and full of sea urchins-nice to look at, but not so nice for swimming.)
The hike back up really isn't too bad--just pack lots of water and you'll be fine.
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