Visiting Eastern Canada (by rail and by cruise) - what not to miss?

Hi We are on a September cruise to Canada/New England which visits the Canadian ports of Saint John and Halifax.

After previous advice from this board, we have booked to fly to Montreal 2 weeks before the cruise and plan to visit Montreal, Quebec, Ottawa and Toronto. We have seen Niagara and I have visited Ottawa for work a few times (like it!)

What shouldn't we miss in Eastern Canada (either within these locations or elsewhere)?

We have about two weeks to explore. We like train travel, so we are looking at travelling on ViaRail between the Canadian cities.

Thanks for any help :)