Pens and paper.
Most writers know to keep pens and a small notebook handy to jot down the occasional idea, but not many realize how valuable these tools are.
Invented long before the computer age, these two items are much easier to carry around than a laptop, are much more lightweight, do not require electricity or a signal and the batteries never die. The pen may occasionally run out of ink, but that’s why you always carry extras.
I suggest using a steno pad or one of those small fat notebooks that fit easily into a purse, tote bag, briefcase or even your pocket.
Using a notepad and a pen is also safer than using an electronic device. I have witnessed people stealing those electronic devices right out of the hands of the users. No one will attempt to steal a notepad and a pen.