A personal pocket computer is something I've found useful for
a long time. My original was purchased in 1991: a Sharp Wizard
Organizer, OZ-7000. A big hulking thing that was, with a full
keyboard, but not QWERTY (later models were), 32kB of RAM, an
expansion card that was PCMIA sized, but before that standard
came out. Powered by three lithium batteries (and sometimes
a fourth inside an expansion card), it still works when I
replace the batteries. As of 2001 it still had data I'd
entered into it seven years earlier. No Y2K problem, either.

These days is is a Palm VIIx, which is smaller, but still a
bit bulky.

I also carry a pen, a sharpie, a Leatherman (the original
configuration, which I find to be the best match of compact
and full-function), my keys (nothing fancy on the chain
these days, I used to keep a pin from a fire-extinguisher),
something to keep track of time (presently the pager my job
provides me), my wallet, and a hankerchief.