We spent several days based in Saint John on this trip. We stayed downtown near the harbor which is a confusing mess of roads and bridges but after a day or two we sort of learned our way around. Saint John fills up a couple times a week when a cruise ship arrives in port. Once they leave at night it seems deserted by comparison.
This view over the harbor is from Fort Howe.
In the center of old Saint John is a nice set of parks which contain the old burial grounds of the city. There are some rather old stones here, many from prominent loyalist families during the American Revolution.
Just across from the old burial grounds is the upper entrance to the City Market. This is one of the more interesting 'attractions' in Saint John and a great place to eat lunch.
The market is a long crowded hall with a lot of vendors and a couple full service restaurants thrown in. You can get anything from a good lobster roll to a mediocre lobster roll in here. Try to avoid the mediocre ones.
Down at the harbor front it's a lot of what you'd expect from a cruise port: shops, restaurants and a large installation of fake people.
We can't cover Saint John without spending a bit of time on the reversing falls. First of all, the reversing falls aren't really a falls. It's just another interesting demonstration of the tidal power of the Bay of Fundy. At low tide the water rushes out and the Saint John River looks like this. At other times it achieves a sort of equilibrium and is relatively calm. At high tide, water rushes into the river and a different set of rapids appear. It's a rare phenomenon but not exactly awe-inspiring.
Here's the view upstream through a series of small islands. There are apparently jet boats that do rapids tours although we didn't see any despite stopping here several times at all sorts of water levels. Elsewhere in Saint John we also visited the Cherry Brook Zoo. Now that we have a small child we will be visiting the zoo in every city on the planet. The Cherry Brook Zoo is much like the falls: good for a stop if you're in the area but not worth a long drive.
Fredericton is about an hour from Saint John depending on the road you select. There's the fast road, the slower but equally reasonable road and the longer, more scenic ferry-laden route. We did return once via the ferry route just to try it. It's perhaps a little excessively slow but the islands in the Saint John River are interesting places to roam around if you have the time.
Fredericton itself is small and compact and surprisingly lively.
The main attraction for us near Fredericton was Kings Landing. No, not that Kings Landing with the Lannisters in it. This is the recreation of a loyalist settlement in 1800s New Brunswick. One of those living history museums with farm animals and costumed re-enactors and a blacksmith. For those US residents out there, this is an interesting view of the other side of the Revolutionary war - the side they don't really teach much in our schools. It's also really scenic in the fall.