This HP 200LX is one of my favorite pieces of tech. It has instant on (right back where you left it) and I only charge the batteries a few times a year. It runs any CGA compatible DOS software. There is nothing today that is more reliable and useful for distraction free or off-the-grid pocket computing.
Over the years, I’ve used this to practice programing on the subway (using the ancient Borland Turbo C++ compiler), keep the books for a small business, write notes and memos, track recipes, and so on. For one of many obscure-off the grid uses, it can run a DOS program that calculates accurate sight reduction (replacing volumes of books) for off-the-grid navigation using a sextant. For outdoor applications, the reflective screen looks great in daylight.
I’ve enhanced the device by replacing the PCMCIA hard drive with a rugged Compact Flash card (in a special adaptor) and by installing “Software Carousel”. This amazing DOS program lets me cycle through several different save states with a keypress. For example, I can jump from a Word document to a Lotus spreadsheet instantly.
As a busy parent, I often go to this device to take a note because it is faster than using my cell phone or computer. This device continues to have a cult following since nothing has replaced all of these capabilities in the last (almost) 30 years.