How about wearing a beged with arba kanfos and tzitzis? How about eating a meal in the sukkah (when not required)? (Yes, I know those aren’t 100% equivalent.)
How about many forms of tzedakah and chessed?
How about mitzvos aseh shehazman grama for women?
I don’t know why the concept of not being obligated but receiving s’char if done is hard to understand. The semantics of the word mitzvah doesn’t bother me a whole lot. Even according to the mechaber who doesn’t allow a brachah saying “v’tzivanu” when optional shouldn’t have a problem with casual use of the word, and certainly not the concept.