I am interested in mechanical pencils that are built to perform different functions. For example, here are some “perpetual calendar” mechanical pencils:
The idea is that by rotating the month part relative to the week part, you can line up the days of the week that correspond to the numerical dates for whatever month it is. Once set, you have a whole little calendar right there, on your favorite writing instrument! Never mind that the month always has 31 days, and you can’t make notes or even mark the date on the calendar, and it is messed up if the parts happen to shift.
The bottom pencil is made in Japan. It is also a “drop pencil,” since depressing the top button allows the writing point to drop out of the body. These instructions came with it:
Another interesting calendar pencil is shown in this post.