Hi vaneska, and welcome to cruise-chat!
We've been to St. John several times during cruises, and it is a favorite port. The link Lyn gave you should give some ideas, and is the main one I use to plan our own trips there. From what you described, I think you might like the Reversing Falls Rapids speedboat. Most lines offer this as an excursion, or you could book it on your own.
There are also some fabulous walking tours on the site Lyn referenced. We've done three of them, and I highly recommend them if you're into history.