Canada's capital city has a wide range of trees with leaves that change to brilliant oranges, reds and yellows in September / October. Peak season for fall foliage tends to be toward the end of September. Check the Ontario Parks Fall Colour Report
and look up "Fitzroy," which is the provincial park nearest Ottawa.
Ottawa's Rideau Canal (pictured) provides a lovely walk around the city and way to take in the fall colours.
In addition, 15 minutes outside of Ottawa is Gatineau Park - 200 km of winding trails that provide a great venue for leaf peepers.