I use this a lot, but it's such an easy POD. Petropavlovsk falls during the Crimean War, Britain nabs it in the peace treaty.
A small naval station is established, a trade post and a trickle of settlers start flowing in. It's administered from Victoria, British Columbia as sort of a quasi-dominion that straddles the Pacific. Winds up as a Canadian territory, and limps along until mining starts being a concern in the area and the Siberian mountains.
It probably has a far bigger effect on Canadian foreign policy than anything else. Canada was intensely and irrationally racist to the Japanese and having something that almost directly borders them is going to amp it up to 11. Japan may also wind up with all of Sakhalin now that Russia lost their (at the time) major outlet on the Pacific.