The 17-55/2.8 should suffice for both landscapes and portraits; wildlife is a little more tricky, since 55mm on a crop is still a little short. Since weight is always a concern while trekking, I would recommend either a 70-200/4 IS or a 135L.
Edit: Just to respond to your question about the 24-105 or 24-70, both are excellent options on a full-frame camera, but the focal lengths are sort of "neither here nor there". 24mm on a crop body gives you a FOV equivalent to 38mm, which is not that wide. If you can live with that, then the 24-105 is a great travel lens to bring along. THe 24-70 is much heavier and I won't recommend it since one of your primary considerations is weight.
I used the 24-105 on a 50D for about a year, and it served me well, but I don't usually shoot landscapes wider than 35mm so it made very little difference to me. This is something to keep in mind.