If you are on a space budget, weight budget and even a price budget, this is the lens that I highly recommend.
The reasonably wide max aperture of f4, means that you can get creative with low light shooting and night photography. The focal zoom range of 24-105 means that you can carry just the one lens for your entire trip whether you are shooting with a crop sensor camera or a full frame.
The price, albeit not cheap, is much more affordable than similar lenses in the f2.8 range. Despite being less expensive, the image quality from this lens is outstanding and price means that you will be less likely to fear taking the lens out hiking, sightseeing and exploring.
Canon invented this lens and now Sigma has released a robust version in its popular Art series for Canon and Nikon mounts.
I recommend this lens if you are looking for just one lens to take travelling.