Hi Joanne,
You'll be coming at a very good time! By late April, lots of migrants will showing up (subject to local weather), with new species appearing on a daily basis - lots of sparrows ("buntings") and icterids, and just the "leading edge" of the wood-warblers.
Agree with the BruceW. about the Outdoor Ontario site, that's a good resource. Within T.O. itself, the Islands are a very good "migrant trap" in both spring and fall; I actually prefer Tommy Thomson Park (a.k.a. the Leslie St. Spit), but it has the considerable drawback of only being open on weekends.
A couple of places that are very good for bird, and just a bit out of town (i.e. you could make any easy day trip of it) are Thickson's Woods in Whitby and Oshawa's Second Marsh (both just east of the "Big Smoke").
If you're going to make an overnight visit somewhere, all the parks mentioned above, along the shores of Lakes Eire and Ontario, can be excellent, but my personal recommendation would be Long Point; it's easy to reach from Toronto, well-birded (thanks to the LPBO), but not crowded. The Carden Plain, up near Lake Simcoe, is an excellent place for "field" birds, but I don't know how many of these will back when you're here, as it looks like it's going to be a very late spring!
PM me if you want any specifics about where to find these places.
A canoe trip could be possible if it's not too cold. Target species includes Snowy Owl, is that possible? Are there any good migration watchpoints?
I suspect it will still be too cold for going canoeing, when you arrive; about Snowy Owls, I would imagine that they'll be gone by then.
Peter C.