I really dug my HP 28S, back in the day. I distinctly remember:

- Taking a required-but-boring course called "Engineering Economics", which consisted of moving money forward and backward in time. Many of the assignments required interpolating on tables in the back of the book. I programmed my calculator to just evaluate the functions directly. (We're talking about 1992 here!) Consequently, I went much, much faster than most of the other students in the class, particularly on exams.

- Getting bored one afternoon, feeling in mood for procrastination, and realizing that HP allowed you to push a function on the stack and later evaluate it. Anonymous functions? Lambda! I proceeded to crank through the first chapter of Abelson & Sussman, just to see if it could be done. It could.

- And then there was the time that my calculator battery fizzled and all that crazy functional programming stuff got nuked. I was quite proud of that work. Sigh.