The least expensive way to get from Pearson International Airport (YYZ) to downtown Toronto is Airport Express, which runs every 20 minutes during peak hours and costs less than $30 for a round trip. The Express makes eight stops on each circuit, all of them downtown. The northern-most stop is at the Delta Chelsea Hotel, which is located right in the middle of the festival area and is within short walking distance of several popular hotels. Total time from the airport to the Delta Chelsea is 45 minutes to an hour, depending on traffic.
I've only had one problem with Airport Express in the past. During my first trip, I had trouble finding the pickup point at the airport. These directions apply to international flights that land in Terminal 3: After making your way through customs and retrieving your luggage, follow the Bus, Van, and Transportation signs. Take the first door out of the airport and turn immediately to your left. The Airport Express stand is located curbside at Area #25.
You can preorder your fare online, but I don't see any real benefit in doing so. The Airport Express stand is always tended, and they accept cash and all major credit cards.