The 18-135 is a reasonable lens and worth a look at - my advice would be to go to the shop with your camera and see if you can try one of the display models, take some shots and see if you are happy with the results.
I used to have the Canon 28-135IS lens and was very pleased with it, for me, a great walk about lens / focal range, but people differ, what 1 likes another dislikes.
I don't like the 18-200 type of lens, I can see the benefit for if you are going away on holiday, but I think the compromise on image quality etc is too much...
Another lens to look at, to match in with your 18-55 is the Canon 55-250IS, a very good price and very nice lens for the price, my daughter has one and loves it, she gets very good results with it, but this isn't going to help / solve the need to swap lenses...