This year I will be in Toronto during Thanksgiving. Will I notice at all when behaving as a tourist? Are tourist attractions closed, do shops close earlier or at different times. Is it better to avoid/visit certain places?
Like American Thanksgiving, Canadian Thanksgiving is largely a family affair. You won't find outdoor celebrations, fireworks displays, or anything like that to the same extent that you would on July 1st or Victoria Day. However there can be major events coinciding with Thanksgiving. If you happen to be in Kitchener, Ontario, you can take part in the second biggest Oktoberfest celebration in the world (the biggest being Munich of course) with a parade, massive beer tents, barrel racing, and more schnitzel and sauerkraut than you know what to do with.
Canadian Thanksgiving is also somewhat tied to harvest celebrations, so you will find farming and pumpkin related events going on. Here's a list of possible activities if you happen to be in Toronto.
A lot of things will be closed on the Monday. Schools and anything government-related, supermarkets and most stores. Some restaurants close but some do not. Most tourist-related things are open, although bear in mind this is outside the tourist season for many parts of Canada. Here is a somewhat better list of what is open and closed.
TL;DR What's open? No government, few stores, some restaurants, most tourism. But check.