IMO, the trinity for going on a landscape shoot is a wide, mid, and long setup. For me on a FF that\'s the 17-40, 25-105, 70-200 +1.4tc. For a 1.6 crop body you have the wide covered and since you only have funding for one more the decission you have to make is how do you generally see ? Given you only have a wide angle it would appear you see see wide.
There hasn\'t been indication of budget but I will tell you that in the long run you will spend less if you get \"good glass\" up front. Otherwise, you just keep respending as you realize what you have is no longer acceptable. If you can I would get either the 24-105 f4 IS or the 70-200 f4 IS.
If these are not in the cards, by a used 28-135 IS. It has its limitations in quality vs L-glass but will probably fit your current needs. If you stay in photography this will be one you eventually discard.
BTW: here is view of some of the sights you can expect to see
