Hopewell Rocks are definitely worth a visit if you have the time. There's just something awe-inspiring about walking on the ocean floor in the morning and then kayaking in the exact same place that afternoon. Plus the caves and rocks are actually quite something to behold. The Reversing Falls in Saint John are okay, though probably not exactly what they sound like. It takes a few hours for the tide to actually reverse, but there's a great restaurant there, and lots of new lookout points and a walking trail (Harbour Passage) that takes you along the harbour to the uptown area, where there's the City Market and lots of inner-city parks. There's also Harbour Station, former home of the Saint John Flames, though I doubt anything will be happening when you're there; if anything, there might be a QMJHL game. My biased and narrow $0.02
