Dennis, St John is great, one of my favorites. Peter has given you some good advice.
I'd add that the snorkeling is good for these days in the Caribbean. It's not what it was 20 years ago unfortunately. If that appeals, hike along Leinster Bay toward Waterlemon Cay and swim around the small island for colorful corals and tropical fish. Also, it's worth a long lazy swim to look for turtles in Francis Bay along the northern rocks. The well-protected beach there is probably my favorite for its glassy water and pelicans fishing. As my 3 year old granddaughter said, "This is not a good beach! There are animals on this beach!"
Hiking is good as well and is necessary to reach many of the best beaches such as Salomon. The national park runs the reef bay hike which is very enjoyable. The North Shore beaches are all breathtaking but don't neglect the wilder south shore accessible with a 4wd. I love the wild Salt Pond hike out to the southeastern point.
It's a very laid back place. If you think national park, you'll get it. There is plenty of civilization in Cruz Bay but it's US and Caribbean, not French.