Waterbrush pens are great for  travel because you don't need to carry a separate container for water!
 
A water brush pen is a paintbrush that has an empty barrel that you can fill with water. When you gently squeeze the body of the pen it will distribute water into the bristles so that you can drop it onto your paints to activate them and then start painting! They are great for things like gradients because you have a built in reserve of water to squeeze and add more and more water to your brush while covering the page.
 
Please note that the barrels of these unthread in the opposite direction than normal. So to unscrew them twist right, and then to screw it back on you twist left!
 
This set includes 6 brushes, 3 round brushes, and 3 flat brushes. The brush sizes are:
size 3 round
size 6 round
size 8 round
size 4 flat
size 9 flat
size 14 flat