I use 15-85mm as my travel lense for outdoors. Overall, excellent lense with sharp and crispy image. Also I always take my cute litte canon 50mm f1.8 with me for indoor images.
REMEMBER, traveling is all about light package and move around fast. Take care of your main lense and treat your 50mm f1.8 as disposable lense; you will have lot of fun like I do.
PS, I fly out 7 times a year and weight is a really important factor to me. Always pack light before fly.