Summer of 2007, I went on Carnival Victory's 5 day trip to Canada from New York. It was fun, though I thought it was too short. I went in the middle of August, there was a heat wave in New York, but I was cold in Canada.
There's a bunch of random things to do in both cities. Back in 1917, there was a big explosion in Halifax, I heard it was the largest man-made, non-nuclear explosion ever. There are artifacts on display from that. There's also stuff from the Titanic disaster, since many of the salvage ships back in 1912 sailed into Halifax.
Past Cruises (all except one from NYC)
Carnival Triumph to Canada: August 1999
Carnival Victory to Canada: July 2000
Celebrity Zenith to Bermuda: July 2001 AND July 2002
Celebrity Galaxy to Caribbean (from Baltimore): July 2003
Voyager of the Seas to Caribbean: July 2004
Crown Princess to Caribbean: July 2006
Carnival Victory to Canada: August 2007
Carnival Miracle to Caribbean: May 2008
Staten Island Ferry, I mean what else in NYC is free?
I did a Canada/New Englad cruise on Holland America a couple years back... It was actually really nice!! The ports were cool and historic and stuff, and there were actually a lot of teens onboard! Halifax was my favorite... mostly because there were really hot guys who would pull you up the hill to the fort in a cart =]
NCL Dawn 6/21/06 Bermuda
10/30/08 X Constellation Caribbean
Costa Fortuna 3/08 Eastern Caribbean
Costa Magica 2/07 Western Caribbean
HAL Noordam 11/06 Eastern Caribbean
X Zenith 6/06 Bermuda
HAL Zuiderdam 5/05 Western Caribbean
HAL Maasdam 6/04 Canada/New England
CCL Triumph 12/03 Western Caribbean
CCL Victory 11/02 Eastern Caribbean
I did a 4 day Carnival Victory cruise to those ports a few years back. On the way up it was cold and stormy, not very very bad but cloudy skies and rain, rain, rain. When I got to Canada it was cool, with sunshine in Halifax and then in New Brunswick it was rainy and cold. It was in July too so just be prepared to deal with the weather because it is not pleasant. Also those cruises are very short, so you may not meet as many people and the trip will be over just as you get "settled" on the ship.
Personally I would take a 7 day to the Caribbean or at the very least the Bahamas/Florida.
Past Cruises: 10
Aug 2000 7 day Celebrity Horizon, Bermuda
Jul 2001 4 day Carnival Victory, Canada
Jul 2002 7 day NCL Sea, Bahamas
Jan 2004 11 day NCL Dawn, Caribbean
Apr 2005 7 day NCL Dawn, Bahamas/Florida
Apr 2006 9 day NCL Dawn, Caribbean
Apr 2007 12 day NCL Dawn, Caribbean
Jul 2007 8 day NCL Spirit, Bermuda/Caribbean
Mar 2008 7 day NCL Gem, Bahamas/Florida
Apr 2009 13 day RCL Explorer, Caribbean