Cape Berton native living in Halifax here. You should have no problem visiting halifax, Cape Breton and PEI in a trip that short. The Cabot Trail and the Cape Breton Highlands are better than you imagine. If you only plan on driving the trail, I'd bet you change your mind when you get there. It's the sort of place you'd want to slow down and enjoy for a while. The other must see in Cape Breton is the Fortress of Louisbourg
http://www.pc.gc.ca/lhn-nhs/ns/louisbourg/index_e.asp Allow youself a full day for this and you wont regret it.
Charlottetown on PEI is where we often head for the weekend. It's one of nicest smallest cities I've seen anywhere. Beautiful architecture, perfect size to wander around on foot, just really really nice. We don't care much for the Cavendish/Green Gables thing. There is a really nice beach there, but we find it touristy tacky at it's worst (or best depending on your interest i guess)
Halifax is always full of tourists in the summer, so I guess they like it
Living here it's hard to recommend things to see. Just head down to the waterfront and wonder. There are many nice restaurants, pubs (The original Granite Brewery now known as The Henry House is an institution among the pub cognoscenti), great museums and galleries... Stuff you find in any coastal city. The public gardens (formal Victorian garden downtown) and Point Pleasant Park (carriage roads and walking trails at the south end of the city) are both nice places to get away from the tourist thing and slow down a little. One other nice thing about Halifax is how central it can be for day trips to nearby places like Peggy's Cove, Lunenburg, The Annapolis Valley, etc. Just grab a map and hit the secondary roads.
People have been mentioning cost, but I find that things like hotels and restaurants are on the cheaper side here. Certainly much less than the US.
I guess my advice is get guidebooks and make some initial reservations, but leave some time open for wandering and happenstance It's that kind of place.