Exchanging money in the airport is easy. With the continuous arrival of international flights, the currency exchange windows stay open 24 hours a day. I have never had trouble with ATM machines in BKK either. ATM cards work easily and might give you a better exchange rate than cash exchange however, for the small amount of money you are describing, there won't be much difference.

As a rule of thumb, never try to obtain Thai Bhat in Canada. You'll find yourself facing terrible exchange rates and service fees. Also, be sure you get rid of your Thai currency and exchange it back into Canadian dollars before you leave Thailand (unless you want to keep a little as a souvenir).