I would keep in mind that in place of an ultra wide angle, multiple shots merged with panorama stitching software can do a very good job too, and save on the hassle of carrying additional lenses. If there is a down side to the 15-85, it's the slower aperture that might make interiors more difficult.
I'd still be inclined to go light, especially since photography isn't the primary focus of the trip. The 15-85 should handle 95% of what you want to shoot, and the IS on the lens and the ISO handling of the 70D will come in handy for some of those less well lighted inside shots.






