A resident of the US needs a tourist visa or other immigration document to visit Canada for a couple of weeks?

You don't need anything

From the website: Most people need a visa OR an Electronic Travel Authorization to travel to Canada – not both. Some people may only need their valid passport. If you’re a Canadian permanent resident (PR), find out more about travelling to Canada with PR status. Answer the questions below to find out what document you need to travel to, or transit through Canada.

You need your "Permanent Resident" card and to for your return in USA , birth certificate or passport you had/have ... Just to be safe!