Great Kayaking on Summit Lake 14km south of Nakusp on Hwy 6 towards New Denver. There is a Provincial campground on the lake, but it’s an old campground and a 40ft trailer would be a tight squeeze. There is a private campground just before Summit Lake Provincial campground that you could fit in no problem.
There is a nice village of New Denver campground right on the shores of Slocan Lake again a Great Lake for Kayaking, plus that hike up Idaho Peak BC is right there.
July is a busy time of the year for getting a site in New Denver but there are other campgrounds in the area.
There are more campgrounds over the hump in the Kaslo area on the shores of Kootenay Lake, plus this kids might like the tour of the paddle wheeler Moyie.
4 days isn’t enough time, but a short couple days just may give you ideas for a future trip to the West Kootenays.
What ever you decide will be great and Enjoy your vacation in Canada.
Soup.