When traveling, which I do often for business, I take a mini art kit with me. While I don't usually have tools, having some 3X5 cards and colored pencils allows me to do design work and capture inspiration when it hits. The cards and pencils make up the mini art kit and is easy to carry. The bit of color from the colored pencils makes the designs come to life and allow for color planning. When I get back to the studio I add the designs to my inspiration board and start prototyping. The inspiration board ends up being a design journal as I can see how I've been inspired over time.
P.S. These pictures were all taken with my iPhone. My first attempt at using the iPhone and editing software. I know, I know, behind the times. LOL