I don't know about you, but variable aperture really annoys me.
In any case, have you considered renting?
One thing I kind of regret about my last trip to Canada was that I only brought my Tamron 17-50, which was great for portraits of the family, but almost useless during a tour on a boat. If I go on another trip again, I would think about renting a (for me, Canon) 70-200 f/4 lens just so I have a good telephoto to swap into.
I'd rent this simply because I cannot justify paying over 1k for a lens that has extremely limited use (in my situation at least.) However, it is a terrific lens.
Short of that, I would say that 55-210 is a good complement (I believe they're normally intended to be used together anyway.) Going wider is great, but it really depends on what you're shooting. For most shots, 18mm should be fine if you can take a few steps back, but if you're sightseeing, you may not be able to walk up very much closer to something.