This was the simplest, most practical, most directly usable purchase for Dollar Store Dungeons: a set of mechanical pencils.
There’s not much to say about these, really. Just that people often don’t seem to have writing instruments, or enough writing instruments, when gaming. Having extra pencils around is pretty much always a good idea.
I’ll toss these in my GMing kit and will probably, eventually, have lost them. But, again, they were just $1.