Quebec
 

My husband and I are considering a short trip (4 nights) to Quebec City at the end of September or very beginning of October.
Any suggestions for sightseeing? We would like to do a whale watching cruise one day but worry that it might be too cold.

Here is a trip report I wrote in 2011 with some suggestions
https://www.fodors.com/community/can...-visit-897235/

We went back last summer when the Tall Ships were in Quebec City. This time in addition to seeing the ships we went to the Musée de la civilisation. This has an extensive regular collection and offers many special exhibits.
https://www.mcq.org/fr/

Another good activity is the ferry boat ride between QC and Levis which offers spectacular views of the old part of the city. There is a brew pub on the Levis side, the Microbrasserie Le Corsaire.

I am not sure about whale watching at that time of year.