(cont.) Then there's always a cheap spray bottle with water, because I'm a lazy person who hates to wet the paper with the brush, and it has the additional benefit that I can keep areas wet for longer periods of time without disturbing the paint. And masking rubber for masking, if I ever plan on keeping areas white. Which I don't. But I sometimes bother to take that bottle with me anyway.
And there's always a paper tissue I clean my brushes with, soak up excess water or color from the paper.