I just joined these boards earlier this week, but I saw your post and had to reply!
I'm from Eastern Canada, and have been to all of the ports you have mentioned. I know Charlottetown, PEI, EXTREMELY well, as we have a summer home in the province.
Anyhow, I took a look at the Canada/ New England shore excursions on the HAL website, and here is what I would recommend, based on my knowledge/ experience/ opinions and the fact that you like knowing the history of ports (I should add that I am a relatively new cruiser- my cruise in March on RCL will only be my 2nd, but I have travelled a lot through Europe and North America. I also work for a national historic site as an interpreter, and we have given tours to many, many cruise ship passengers).
*Quebec City- If you are up for a bit of walking, the "Quebec City Walking Tour with Tea at the Chateau Frontenac" looked really good. The Chateau Frontenac is definitely a site that is not to be missed, and Old Québec is very charming. It's only 3 hours long, and you're in port for 9 hours, so try to get a river cruise tour in too. They're not expensive at all, and are a great way to get a different perspective on the city. My second choice would be "A Taste of Old Québec: Walking & Wine Tasting".
*Charlottetown- If you are truly into history and want to experience the essence of Charlottetown, I would suggest "The Charlottetown Story: A Walking Tour". I've seen these tours going on during the summer, and people seem to be enjoying themselves. Heck, I've been tempted to join in!
However, if you’re not planning to visit PEI ever again, I would say to do "Island Drive & Anne of Green Gables". I worked in tourist information in the province for 4 years, and this tour has the stops that I always suggested to people who wanted to see "everything". I could go on and on about PEI, but I'll leave you with these. Feel free to ask me any detailed questions about Charlottetown- good restaurants, shopping, etc...I know it all!
*Sydney- This is a hard one! However, seeing that you have 10 hours in Sydney, I would suggest "Baddeck & the Alexander Graham Bell Museum". When I was last in Nova Scotia, this museum was one of the highlights of my trip. The tour is only 4.25 hours long, so you will have lots of time to walk around Sydney itself afterward (and possibly take in the "A Guided Walk in Old Sydney Town" tour too- the excursion site didn't say how many times a day this tour is offered). If you have any questions while you're meandering around, don't hesitate to ask anyone- we Maritimers love to talk! My friend who made the following comment was right on the mark- "Take a map and stand on any street corner. I will 100% GUARANTEE you that in a 5-minute period, you will have at least 3 people asking you if you need help".
*Halifax- I'd do "Garden & Garrisons--A Relaxed Walking Tour". This one seems to take in most of the historic sites in Halifax, not to mention the gorgeous Victorian architecture. The only major one I noticed that it does NOT take in is the Halifax Citadel (if you look at pictures of the city, this is the place on the hill with the big white clock). HOWEVER, seeing that you have 9 hours in port, you could easily get there after the "Garden & Garrisons" your ends- the Citadel is quite close to the dock area.
*Bar Harbour- One thing I would highly suggest doing here is visiting Acadia National Park. My family has hiked here many times, and it is a very beautiful place. Looking at the excursions offered, I would suggest "A Walk in the Park (Ocean Trail Hike)". If hiking isn't your thing, but you don't mind strolling at a leisurely pace, "Step Back in Time: A Victorian Walking Tour" looks good. Bar Harbour is a quaint little place, and should be bustling in August (I've only ever been there in November, when most travelers are long gone).
I hope this information helps you out. I suggested the excursions not knowing your physical abilities, so some of them might not be appropriate. Again, if you need any more information at all, I am on these boards just about every day and will keep an eye on this post.