I know that for the 2 Quebec Spring Break weeks (late Febuary/early March), there are always correct "last minute" offers from Montreal (and GREAT deals out of Toronto, if someone would want to drive...). But that means not being picky, and take what's on sale (thought I could do it year before last, but then started to look and fell in love with a great place...). Is Ontario all on break at the same time? That would make it difficult. Otherwise, if it is spread out over 2-3 weeks, with the numbers of flights out of Toronto, you should be able to find something. That is based on past years experience, but with the rising price of fuel, I just don't know if it still will be the case. There might be less flights going out than before. The winter (cold, snow) also influences the sellof offers, like this spring, prices didn't go down in April like the year before, since we had so much snow.
Hope you'll enjoy your vacation, I love all-inclusive, it's very relaxing, it's not a "trip", but a vacation. Nothing to think about, organize, or decide, just "Eat, drink and be merry". I went to Cuba in April for the last 2 years, once at the place I fell in love with, the other at a great last minute deal (750$ for a 4*!), and I fully intend to return next spring!