The OP will have to decide what kind of a drive they want. But I agree the OP doesn't have a handle on what's involved. And I notice I misposted Toronto to Halifax, not C'town in my earlier reply.
You can "get there" in 18-19 hours, but which tourist would enjoy that and how would that make their journey enjoyable. Even driving 9 hours for 2 days, minimal stops, eating on the go would not be something I would suggest to a tourist who is asking <<Cities to visit on way - what to see>>. My advice to the OP, as I try to think like a tourist, is to take time on the journey at 5 hours driving each day allowing time for 3 hours of sightseeing, eating each day. This would make 4 days. This would leave 4 days for PEI. Decisions, decisions.
The number of things to see between Toronto and Charlottetown are too numerous to mention. Ottawa, Kingston, Gananoque, Montréal, Québec City, Gaspésie, NB and finally PEI. Some in one direction, some on the way back.
But my #1 advice would be to fly and spend those two weeks on the ground, with a rental car, in the beautiful scenery of PEI and Nova Scotia.