Henri - do you remember the pages I did recently of Matt and I on the tall ship? You'll sail into Halifax that way, past George's Island, to the waterfront (I had the huge 1812 pic there). There are a ton of little shops along there.
You'll want to hit up the Citadel for sure - and they don't close for the winter until the end of the month. http://www.pc.gc.ca/eng/lhn-nhs/ns/halifax /natcul.aspx If they're still doing it this time of year - and they likely are, with a ship in port - they have a reenactment program that's really neat.
This is the 100th anniversary of the sinking of the Titanic, and there are a bunch of events happening since there's a Halifax connection. I know the Maritime Museum of the Atlantic is doing a special program until the end of Oct - and that's right along the waterfront.
As far as tours go, check into the Halifax Harbour Hopper - you drive around in an open vehicle that can go on the roads or on the water.
There's also the public gardens - that's only a short jaunt from the citadel. Very pretty! Right across the street is a Robbie Burns statue, too.
Pier 21 is where immigrants to Canada came through from 1920s on and military left for the world wars - it's something to see, really, and right on the waterfront - I'd assume your cruise ship will likely dock in that area. I know they have a huge display on war brides specifically and WWII as well.
There's also a brewery in the area that does tours. If you need a recommendation on a place to eat, I can give you one as well. I'll PM you links to the local scrap stores once I'm at home on my own computer.
Edited to add: And considering you've got that much time, you might want to consider going further afield and visiting Peggy's Cove or, if you're in an agricultural mindset, the Valley (where I am now) is only an hour away. You'd pass through the birthplace of hockey (windsor) and we're in the middle of a pumpkin people festival. Highest tides in the world, too....y'know you wanna!