My favorites:

Caneel Bay - great ruins, beautifully manicured grounds, nice beach. Ask about Turtle Cove for Sunday Brunch.
Cinnamon Bay - great hiking trail across from entrance and just a gorgeous beach.
Francis Bay - another wonderful long stretch of beach just past Annaberg and Leinster - not so much a snorkel beach, just long stretches of smooth white sand.
Annaberg Ruins and Leinster Bay - wonderful views from Annaberg of the Narrows and Tortola/BVI across the way, beautiful old windmill and sugar plantation ruins. They have demonstrations during season so check the Park schedule. There is great snorkeling at the end of Leinster Bay - wear good shoes as rocky beach.

Trunk Bay has a very nice snorkel trail but I always find it too crowded, especially in season.
When you pay to get in Trunk, you can visit Annaberg, on same day for free and the same is true in reverse.
It's a $4.00 fee which is minimal.

Off to the the east end on the north shore road after Annaberg.
I always enjoy Shipwreck Landing for lunch and the shops there.
My favorite bay has to be Lameshur. You need a good car, sometimes 4WD if its been raining. It is the farthest point you can drive on STJ.
Be sure to keep and eye out for our small local deer as you drive along the road to Lameshure. Wonderful snorkeling.
You can also hike to the petroglyphs and the ruins at Reef Bay from this location.
Bug spray, sunscreen, water and good shoes to do the trail!

Check out the Visitors Center at the National Park Building - they have hiking maps for all the trails and a list of scheduled events and tours that might interest you.

Don't forget to check out Lime Inn when you get back to town for lunch or dinner. They have wonderful Hot Fudge Sundays!

There's lots to do! Day Sails, fishing, SCUBA diving, kayaking, horseback riding, hiking and checking out a couple different beaches everyday.
Don't forget Honeymoon Beach!
Enjoy!

Thank you so much, Alana!! It's so kind of you to take the time to share this information with me. I'm looking forward to my visit! I guess I will check out some other message boards to see if I can get some additional input. Hope everyone has a great day!

Have to argue about "Francis Bay..not so much a snorkel beach..."
On the contrary, it is our favorite for snorkeling. Have always seen turtles and it was the 'home' for the manta last year. You need to head north along the beach ~ 100 yards. Snorkel out ~ 30 yards where you come across the sea grass and keep an eye out for the turtles. Last year we thought it was going to be the first non-sigthing and all of a sudden we saw a huge one. We were so taken by it we didn't notice the the other five about 20 yards away until my son got our attention.
If you carry on north close to shore swimming toward Whistling Cay you will come across rocks & coral with eels, barracuda and the occasional octopus along with the normal cast of characters.

Another recent gem for us is Haulover North which faces BVI. A little 5 minute walk brings you to the pebble beach. Snorkel to the left (north) for very healthy coral. A little rough getting in and out compared to sandy beaches but worth the effort

On both of my two stays on St. John, I've enjoyed Jumbie Bay on the north side. Quite small, but a wee gem.

Tom

Thank you all SO much for taking the time to provide your words of wisdom! I am very appreciative. All the best to everyone!

Enoy your trip!