We started with the Montreal to New Your 7 day cruise last fall, in fact, we stayed on all the way back to Tampa. Before the cruise we stayed at the Bonaventure Hotel, which we loved! BUT, we used to live in upstate New York and visited Montreal often, 20-30 years ago, and often stayed at the Bonaventure then, so it was like 'deja'vu all over again' for us. The suggestion about the Ritz Carlton is great, particularly at the rate quoted! The Queen Elizabeth is also a great hotel, and very convenient if you take my next piece of advice, as it is right on top of the train station, and the bell-hops will (used to, anyway – inquire) take your luggage right to the train.
If you haven't finalized your to-from connection arrangements yet, or if they are changeable, may I make a suggestion? Amtrak runs a daily train, The Adirondack, from Montreal to NYC (and vice versa). It leaves both places in the morning, and the scenery is spectacular, particularly after the border as you travel down the west shore of Lake Champlain with the Green Mountains of Vermont and the lake itself providing wonderful views out the left (port side) windows. The train continues through Saratoga Springs and Albany where it crosses the Hudson River before hitting the next scenic wonder on the right (starboard) side as it runs down the east bank of the Hudson right into New York, going underground about 59th St. on the west side, just above the NYC Passenger Ship Terminal where you started your cruise. Less than an hour before hitting the city, you will pass West Point and then Bear Mountain across on the west bank of the river, and a bit later, go right through Sing Sing prison (at Ossining), although you can't see much except high walls on both sides of the train as it speeds through! Somewhere between Montreal and NYC you are almost guaranteed to see the brilliant fall foliage between about now thru late October. And taking the train from Montreal to New York City will allow you to book round-trip air to NYC, avoiding the need for open jaw tickets. The train trip takes about 10-12 hours, depending on the border delay, and costs ±$60 pp. It’s a very worthwhile and beautiful trip, but at times, it can be a bit slow, like most of Amtrak these days.
We used to go to the Winter Carnival in Quebec City, but we had never been there in good weather. We just took the free bus into town, walked around, and had lunch at the Chateau Frontenac. Peggy’s Cove is the big attraction in Halifax, but just walking into the city on the boardwalk along the waterfront and taking the hop-on/hop-off trolley tour from there can make a very nice day as well. The tour of Acadia National Park in Bar Harbor was one of the best, most impressive short tours we’ve taken in quite a while. Make sure you are on your balcony, or on deck, as you go through the Cape Cod Canal, as it is well worth seeing, and easy to miss (we almost did!)