Hey, I'm from Halifax, but I now live in France. If you plan on starting in Halifax, to get there for Montreal the easiest way would be to fly. Check out "low cost" as in cheaper than Air Canada airlines like Westjet, Canjet, etc. If you dont fly I believe there is a train from Quebec city you could take. Other than that I would say take a bus, it's a long ride, but it will get you there.
Once your in Nova scotia you can take a bus to most places that arent too obscure, your best bet to see everything you want though is to rent a car depending on how much money you can spend I dunno if this is an option or not but at least check out prices, if you plan to camp at all there are a lot of places in the summer. I'm not too sure about hostels but I know there are a lot of B&Bs and discount hotels around.
Some places you might want to check out include Halifax, Peggy's Cove, The south shore area which includes Lunenburg and Shelburne. If you want to see some whales go to Digby on the Bay of Fundy and also the Cape Breton Highlands are a cool place like Baddeck. The highway routes in NS are pretty good for the most part if you have a car.
Check out the tourism website posted above for more info too...
When you're in Halifax some good places to drink are for Irish pubs - Old Triangle, Lower Deck, Pogue Fado and the Split Crow. Other bars that are good are Pacifico (kinda trendy), The Dome (meat market) but not too bad and open till 4am. The Palace (dance club), Alehouse and the Argyle. They are all within 10 min of each other in the Downtown section so they arent hard to find. Everyone usually starts at some bar then ends after 2 at either the Palace or Dome because they are the only 2 open late.
I'd definitly recommend going to the South Shore, check out the tourism website for more info. And if you have time check out Prince Edward's Island if you can get there. There is a ferry from NS and a bridge from New Brunswick. The beaches are incredible. most popular are Cavendish and Brackley Beach.
Whatever you do, Im sure you'll have a good time. If you change the timing of your trip for any reason I believe there is a tall ship festival sometime (not sure exactly when) and in August there is the Busker (street performers from all over the world) for 2 weeks, they are pretty cool to see.
If you have any questions, fire away