Be sure to check the provincial tourism sites for each province. There is a lot.
Some things I remember from my trips.
Nova Scotia, Canada - Lonely Planet
NB - Bay of Fundy Tides - Hopewell Rocks
tons of historic sites
Tourism New Brunswick : Destinations, Accommodations & Activities
Cavendish Park - Red Sand Beaches
Tourism Prince Edward Island (PEI) - Canada - Official Guide
If you are into National Parks, there are a few between these three provinces. A pass might be worth investigating. The local provincial parks can be very nice too. An Example
Kiji Provincial Park
There are scads of historic sites, lighthouses, covered bridges and such. It really depends on what you are into.
NS has a bit of a wine industry going now so you might find some different things that you can't find elsewhere.
There should be opportunities for music, lobster fests and so on as well. The local calendars can be quite entertaining as well.
If you are looking for time management tips. My personal preference would be to spend the most time in NS. I found NB to be less entertaining and PEI is really not that big. Maybe 2-3 days in PEI (more if you like to hand around on beach). maybe 4-5 days in NB. Tides are interesting but not that interesting and the rest in NS. There is something to see in most regions of that province. PEI is more agricultural than anything unless you are into the carnival atmosphere that surrounds the Green Gables stuff (Kind of a mini Niagra Falls feel) and I found NB to be a bit misleading. The Bay of Fundy Scenic Hwy is about 70% inland for example. Magnetic hill was a bit of tourist trap kind of thing. You also pay good money to walk the boardwalks at Kouchibouguac National Park only to find the ones at Batouche free and much more extensive.