St. John is our happy place. I adore the island and dream of living there some day.
trunk Bay is a beautiful beach but a lot of cruise ships will take excursions there and it's a bit touristy. That said, you can rent water sports stuff and have a great day there.
If you snorkel, go to Cruz Bay water sports and rent gear for the week. Pick a different beach each day and snorkel. Ask them at the shop for recommendations of best places bc it changes depending on winds, weather. Some of our favorites are Waterlemon Cat, Leinster Bay, Maho, Lameshur and Grootpan.
The reef trail hike is awesome. Wear good shoes for it. The petroglyphs are so amazing.
If you can, consider renting a charter sail to the BVI's. Hands down one of my favorite things we did. We had a sailboat take us to The Indians, where we snorkels around. Then we went to the shore of Norman Island and snorkels in some caves there. We had lunch at Willy T's which is a floating boat restaurant. Food was basic but the painkillers (island drink) were so good! There are a ton of different type sails you can do but going to BVI's is worth the trip.
Go to the Star Market once you've dropped your stuff off. They have the best selection as far as groceries go. But be prepared for very high prices. It's super expensive to shop on the island.
Go to The Tap Room in Mongoose junction. It should be reopene by August I think. It burned down a year ago but they were rebuilding. Get a mango pale ale. Best beer ever. Mongoose junction is fun to shop around in also.
Restaurants tend to come and go in St. John but I think Shipwreck Landing is still over in Coral Bay. It's a great lunch spot. The deli in mongoose junction is really really good! Skinny legs is overhyped in my opinion. But ymmv. On the way to coral bay there is a smoothie place called Colombo's. It's at the intersection of King and Center,one red. It's literally a little trailer looking shack. But the smoothies are amazing. You can get with or without rum. My favorite is the lime and coconut. (With rum!)
If you have any specific questions, please feel free to ask. If I think of anything else, I'll come back and add it.