High on my list of "indispensable writing tools," probably just below "my MacBook" and "Scrivener," sits "The Notebook."
Anyone serious about writing should have a small notebook that goes with them everywhere. I've heard many graphic designers, artists, architects, and other creative types from many different fields say the same thing and for a similar reason -- you never know when a good idea will cross your path or enter your head, and you don't want to risk losing it. For writers, it's ideas about characters, plot, dialogue, interesting facts, places, etc etc etc.
It's a practice that many great thinkers throughout history have followed. (Don't let the article title or the linked website fool you...this is great advice for anyone, regardless of gender).
"The list is hardly comprehensive; the practice was so widespread among eminent men that it would likely be easier to compile a list of famous men who did not use them, than did."
No endorsement intended, but mine is an old FranklinCovey binder that I keep filled with lined pages and which I've carried around for a decade now; some people use iPads and iPhones and an app like Vesper or Write for the same purpose , but a cheap $2 notebook can work just as well (so would an iPad or iPhone). No one who says they're serious about writing gets to plead poverty on this one. Find something that works for you and start carrying it everywhere. You'll find pretty quickly that you feel naked without it.