Casa Loma (House on the Hill) is a great place to go and visit. Completed in 1914 for $3.5 million by Sir Henry Pellatt, it's a medieval style castle with a great view from the top floors of downtown Toronto. Unfortunately, 10 years after building it, Sir Henry Pellatt was forced to abandon it.
It costs $10 to enter. It's hours are 9:30-5pm. Make sure to get there by 4pm though as they don't let people in after that. There are plenty of tour buses that run from Downtown Toronto. Our one dropped us off at Casa Loma and you buy the tickets on the tour bus. You can look around really quickly in about an hour, but plan to spend 2-3 hours so you can really spend some time wandering around all of the rooms and the beautiful gardens outside. Make a point to go to the top of the tiny, spindly staircase to go to the top floor of the castle. You have to plan your ascent and descent as there is only enough room for 1 person on the staircase, but at the top you have some great views of Downtown Toronto and the CN Tower.