Thanks for that advice on keeping the camera dry with the Keyhole; I'm making one up to use, too. I'd for sure take the zoom and probably the 2X adapter. That Canon-IS 300mm, if it's like mine (we've got the same camera), is pretty heavy. I, like you, can't afford the Canon 300mm IS lens (It is now "standard" in the two lens packages I've seen.) If your pack's heavier than you want it to be, that 300mm telephoto might be something to leave behind. taking the 2x converter for the rare telephoto shot.
(avatar: raptor, Lake Dillon)