In the Maritimes, I would highly suggest going away from the Transcanada where possible and take a scenic route on the coast. The best of the Maritimes are on the coast while the Transcanada largely just goes through forests. You are going to miss out significantly if sticking to the TransCanada and may as well be traveling in Northern Ontario (I'm sure you won't want more of that).
You can travel via Gaspésie (do the whole thing and see Percé or go through 132 / 299). The East coast of NB is underrated as it's not a big tourist destination and lesser populated than the south but beautiful to go through (Miramichi to Shediac). I would recommend this type of itinerary if you want to see scenery during your road trip while not doing usual touristy stuff.
If you do go south NB instead mainly via the TCH, I would suggest a pit stop at St Andrews (you could do a whale trip there, or at Briar Island in NS if you want). Highly recommend doing the Saint-John - Digby ferry for a fun and unique experience (after spending time at the Bay of Fundy), and a fun driving experience in the northwest coast of NS.
In Nova Scotia, great scenic choices off the path are Lunenburg, Annapolis Royal. Cabot Trail of course is a must. Peggy's Cove near Halifax is a popular spot. While I have never been, Louisbourg would certainly be a great pit stop of historical significance. Also spend a few days in Halifax proper, probably my favourite city in Canada. I always go to Bluenose II Restaurant everytime I can when I am in the city, seriously nothing beats a coastal diner.
Plan out a lobster boat trip based on where you may be and if it is in season. Lots of towns have tours if this is something that interests you. Something like this
https://shediacbaycruises.ca/lobster-tales/.
PEI I can't speak on since I've only been there a couple of times when very young before the bridge.
Newfoundland is deceptively large (Channel-Port aux Basques to St John's is 9 hours), plan accordingly. St John's Signal Hill is a must and one of the most incredible views you will ever see. Ferryland Lighthouse is a great trip as well. Highly recommend spending a night at Quidi Vidi Brewery for a truly unique experience while having great beer.
Ontario you should decide if you want to skip it altogether via TCH, or go see the Falls / Niagara on the Lake. They may be worth the large detour for you. Not much otherwise unless you want to spend some time at Algonquin before getting to Ottawa.
In Quebec, most will be around Quebec City or Montreal: Plains of Abraham, Mount Royal, Old Montreal, Charlevoix, Montmorency Falls.
Most of western Canada has been already spoken about in length here but Drumheller or Dinosaur Park are musts if you never been.
Have fun!
