For the record, I AM Canadian, and I don't know what a Canadian costume is...
In parts of British Columbia, you could wear Asian or Indian clothes, tie dye t-shirts or the safety gear of a logger or someone working in a refinery. In Alberta and Saskatchewan, you might be a rancher, or a farmer, or work on the Oil Sands Projects... Manitoba will find you German and Ukranian communities that have been in Canada for generations, Ontario runs from the industrial/commercial/financial centres in the south to the resource based industries of the north (and the centre of the economic collapse, the southwestern part of the province), in Quebec, the predominant language is French, and aside from large cities, it is the only language spoken. You could dress as a fisherman from the Maritimes (as apparently the rest of the world think that is all that happens in Nova Scotia, New Brunswick, Prince Edward Island, Newfoundland and Labrador.
You could be a seal hunting Innu from the north, or a First Nations person from any province...
More than 50% of Canadians have been here for 2 generations or less.
And many have been here for thousands of years - there are over 600 First Nations in Canada - the aboriginal groups who were here long before Europeans arrived...
We don't have a "costume", we pretty much don't have a "culture"...
BUT... you can always grab a bottle of beer, a back bacon sandwich, put your jeans and plaid flannel shirt on and go as a stereotype...