Vaneska - - I've not been there, however, I usually look for a site specific website to see what's available. You can search to the port name and find what comes up. Chambers of commerce, tourism boards, historical societies ... it's always interesting what pops up on a search. Good luck and enjoy your cruise.
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.
Just wanted to make sure that when you research via external websites, you look for things pertaining to Saint John, New Brunswick.
St. John (or St John's) is Newfoundland - different province, although still the Maritimes. It was one of those annoying little hick-ups in Canadian Geography/History class.
I'm Canadian and I still haven't been to New Brunswick - but I checked out the excursions and I'd probably do the MooseHead excursion just because it's got that east coast pub/entertainment vibe is I wanted a taste of the local vibe in 3 hours.
The real gem is the Bay of Fundy (as you can probably tell by all of the excursions that have Bay of Fundy thrown in) - it's probably considered a must see if you were to go on vacation there - they have the highest tide in the world I believe. It's also probably pretty beautiful.
A trip out to the Hopewell Rocks is an awesome life experience - then again St.John is worthy of a day's visit! We may be on the same cruise- I am out bound on the CCL Triumph on June 11th from NYC myself!