Houseboat!
We just returned from houseboating on the Shuswap Lakes, which is surprisingly warm this time of year, and because it's September, you get the discount rates and nearly the entire lake to yourself! Alex was wonderful the whole trip, he barely fussed, mainly because there were 6 adults looking after him, giving him all their attention.
I would definitely do the Shuswap Lakes again in September, although maybe we'd do a cabin or something stationary, so we can avoid the whole "Where should we beach tonight?" and all sorts of other "where should we go today?" arguments. You just CAN'T leave the boat in one place, even if you have found a great little harbour because you're in a houseBOAT that must travel!
Here's a pic of Alex driving the boat: