We spent 4 nights on PEI and so had 3 full days plus another 1/2 day. There is plenty to see and do - even without being too interested in 'Anne'. We really only saw the eastern half of the island. I think this is about the minimum time I would have spent there. Another full day would have been better.
Nova Scotia is much bigger - and often slower to drive around. We actually spent 11 nights there and only scratched the surface.
Just for the Southern Part:-
We didn't get to Yarmouth so I have nothing to offer about it. You could head north on either the west or east coast. With you limited time I would use the eastern route so you could stop in at Lunenburg & Peggy's Harbour on the way up. You will need to remember, as I said above, that the driving is very slow.
As regards other particular spots. See below :-
Halifax can probably been done in a day itself - we spent 3 nights there as it was the end of our trip and we did drives both south, to Peggys Cove , and north from there.
However we also stopped a couple of nights in Wolfville, which is in lovely scenery - the Annopolis Valley - and full of wineries and has a lot of history nearby at Grand Pre relating to the Acadia's and their 'problems' with the British. You can also drive to Halls Harbour where you can see the full tidal movements of the Bay of Fundy - we went at both low and high
Lunenburg is another lovely and colourful town. We only spent a few hours there and shortchanged it.
Peggy's Cove is an easy drive to/ from Halifax - but get there as early as possible to beat the tour buses. If possible use the coast road though Mahone Bay and stick to the coast as much as possible. Again, very slow going but some of the most beautiful scenery I have ever seen. ( If you watch the TV Films of Jesse Stone" starring Tom Selleck you will see the scenery as it is not filmed in the supposed setting of Maine but around Mahone Bay etc.
Annapolis Royal is also a very historic town with a fort that changed hands between the British and French plus an historic graveyard with night tours - not a ghost tour but a history tour by lamplight with an Acadian descendant.
Now to the north.
Sydney on Cape Breton Island - just drive past it. It is an old mining town struggling to come to grips with its loss of work. There are interesting things there but not suitable for your time restraints.
Louisbourg, near Sydney, has a great reconstructed French Fortress full of re-enactors and artisans. Again, I think you could ignore it with your time constraints.( As an aside, we did stop there for 2 nights to visit the fortress and partake in " The Beggars Banquet" a show with lobster meals, dressing up and entertainment with old French songs. We know that the songs, at least were genuine, as we shared a table with a young woman bought up in the French countryside and she said that they were the songs song at the traditional village fairs of her childhood. All round a bit of tosh but fun.
Then you move onto the Cabot Trail. It was bit overcast when we did it but still some great views. The Skyline Trail was well worth the walk. we spent a night each at the start and end of the northern half of the trail.
Sorry to be so loquacious but you will have to choose your destinations carefully. Even driving virtually non-stop it is a full days drive just to get from Yarmouth to Charlettetown. So I guess what i am saying is that you either need more time or will have to be very strict on what you decide to visit.
Personally I would be inclined to cut a day out of Montreal. Vancouver I just don't know so will not comment.