There's a lot to see and do around here - more than enough to fill 3 days. The NS tourism site: http://www.destination-ns.com/default.asp will give you lots of information and if you call them, they're very helpful. The 'Doers and Dreamers' guide available from them is full of detail about what to see and do anywhere in the province.
You should also look at the site of the Black Cultural Community of NS: www.bccns.com There's a lot of history on this site and they have a museum. They're located in Dartmouth, across the harbour from Halifax.
Have you read any books that are set in NS? If so, you might want to check out those locations. That's just a start. Don't be shy about posting any other questions you might have.