Ooh Gandalf, my Mom lived in Kelowna for a time. Gorgeous place, lots of lovely vineyards around there too Best Pinot Gris I have EVER had. We did Vancouver, Kelowna and then Banff/Lake Louise. Fabulous place Op, am very
just spent 10 days in Vancouver then a week in Kelowna, right on the lake (water skiing, kayaking etc.) with vineyards & wineries - lovely 20+ degrees for the city break then 30 degrees in Kelowna - was perfect & wasted minimal time on air travel during the holiday as Vancouver to Kelowna just over an hour! If you go, the BC wines are wonderful now, but not big enough for export, so the locals keep them for themselves - make sure you try them.
July in cottage country in Ontario is hotter than hell. Very humid and steamy and only bearable if you have a lovely cool refreshing lake to jump into and dock to sit on whilst consuming beer after frosty beer.
Days are warm / hot in August but the nights cool off and you can have some cracking campfires.
"Hottish in August" - yes. Vast areas get baking hot, dry summers...but coastally, not so much. If you do want to stick with Canada, though, the obvious combination would be the BC/AB Rockies, combined with a boozy week in the Okanagen with its beautiful lakes and vineyards.