We love St. John here are some things not mentioned above:
Get the Fun, Fins, and Four Wheel Drive book. It has a ton of great ideas and directions.
We did the Reef Bay Hike through the Park Service. It was about $25-$30 pp and was well worth it. You park at the National Park building in Cruz Bay and they take you up to the top of the trail by taxi. From there you take a guided hike down the trail to the beach where a boat brings you back to Cruz Bay. Along the way the Ranger will speak about the flora and fauna with a stop at the Petroglyphs too. At the bottom of the trail are the ruins of the old sugar mill. Really a great time and very informative.
We also hit Dennis Bay Beach this year and found it to be beautiful. Not crowded at all we practically had the beach to ourselves on a Sunday afternoon. It is a short hike down from the Peace Hill parking lot. We were able to take a cooler and chairs so it's not too tough.
We hit up Spyglass in Warfside Village for happy hour this trip and really enjoyed it. They offer several apps for $5 and have drink specials too. The staff was fun and friendly and the view is great.
We did La Tapa too this trip and really enjoyed that meal.