Hi,
Central Toronto isn't great for sandpipers, at least not compared to what you are probably used to in the Netherlands. You can find some at the Leslie Street Spit and at the beach on Toronto Islands. But, I have never seen great numbers there, and these are areas with lots of people moving around, so wading birds tend to take off rather quickly. The other option is to visit lagoons outside the city, but I have no experience from that, since I don't own a car. Other people on the forum may be able to advice you.
When exactly are you coming? Mid summer can be a little dull in terms of birds. Well, you have all the local residents, which I'm sure will be interesting to you if you've never been here before, but there won't be much migrants.
Let us know when you arrive here (send me a pm), if you want to join up and go for a hike to the Spit or the Island.