These are the Markham Fair grounds. I missed out some of the grass, and the track, but this is pretty much what you get. It doesn't look much like tourism=theme_park, does it? But that's how it's currently tagged: Way: Markham Fairgrounds (43934707)
Strangely enough, it and many others like it in Ontario are tagged as theme parks. Now they might have a midway for a week or so during summer or fall fairs, but the rest of the time, they look like this.
The previously proposed amenity=show_grounds would be better for most of these. The ones that have permanent installations, like Canada's Wonderland, should stay as theme parks, so a little local knowledge is required.