I've been to all of them quite a few times, so here's some input for what's it's worth.
Bar Harbor:
It's a very touristly town and not my favorite coastal town in ME for that reason. But if you get over to Acadia and do some hiking, biking, carriage rides, then do so. Plenty of good local places to hang out in for drinks for sure. The Thirsty Whale is a local hang out. A few of my favorites are now gone/rebuilt. Dog & Pony Tavern is another cool place to hang out. Seems to be pretty local too. It's my favorite spot that I've been to in town. Off the main streets/touristy spots. Stewman's on the waterfront was always a good spot for traditional Maine food, but it's been many years. They tore down the original one and rebuilt. But since it's down on the waterfront, it will be a busy spot. Still very nice view of the narrows.
The best town of all of them is Quebec City. It's one of my favorites cities in North America by a long shot. Very European in scale and overall a cool place to spend a few days. Do the tour of the "fort" - it's actually pretty interesting. Walk down Rue Saint Louis - there are a ton of great outdoor cafes along the top section of this street. Once you enter the "walled" section of the city it's quite a few small shops and little bars until you get down to the waterfront and plaza area. Stay to the left around the green and there are lots of art vendors and more shops/bars. I believe it la Fabrique - a very nice/cool street with quite a few good restaurants and bars. I've spent most of my time at those cafes on Saint Louis and Fabrique to be honest. Did the tour of the fort and tooled around the famous Hotel Frontenac. I am guessing you'll be coming from the opposite direction though - I am coming from the top where the hotels are.
Sydney NC: Not a whole lot going on here. I've only hung out here because the ferry comes in and out of here to Newfoundland. It would be cool if you had the time to take the ferry to the French owned islands of St. Pierre and Miquelon. If that's possible, do it.
Halifax: Still somewhat "industrial" by Nova Scotia standards. The popular beer up there sucks in my opinion, as does most main stream Canadian beer. But the beer scene has gotten better here. Just Google them, but stay away from Alexander Keiths. It's shit. Stick to 2 Crows and North Brewing - there are others too, but don't remember names.
PEI: It's been too long since I've been there and honestly don't remember much of anything in terms of specifics/places. But hopefully someone can chime in that's been there more recently.
Unfortunately cruise ships are taking you to the less than great places of Nova Scotia. I have spent a lot of time in Cape Breton and the south shore where my family has had a house just outside of Lunenburg. The small villages are really nice with some great coast line.
I'll add more as I remember stuff.