As a Wisconsin native, I like to think I know a good brewery when I find one. And after leaving the brew-culture wasteland that is Quebec (there are no tours! of anything!), I fell in love with Steam Whistle Brewery at first sight.
If you're visiting Toronto: go here. The building and surrounding area are absolutely beautiful, and you feel like you're in a little enclave in the middle of Everything. It's right next to the CN Tower and the Skydome and everywhere else you're going, so it won't make you lose valuable touring minutes in transit. There is beautiful art on the walls, everyone around you is happy, and there is free beer.
The tour was short (half-hour?), informative, and worth it to see all the gorgeous rooms. You have never seen such a sparkling clean factory, seriously. Plus, you can get a tour-plus-beer option that results in your having beer for SO much less than it will cost you at the Beer Store. (The tour is $10, I think, and then for $15 you also get a six-pack at the end.) Maybe not feasible if you're going to spend the rest of the day wandering around, but worth considering.