At the restaurant, you are presented with a desert menu. Assuming that you only eat one desert, you have the choice between chocolate mouse, ice cream and
honey crepes. Let's say you prefer ice cream to chocolate mouse, but honey crepes are your favorite. If you eat a honey crepe, ice cream (which would have chosen had you not had crepes) is your opportunity cost.
Another example would be if two women that you fancied,
Sarah and Alice, asked you out and you said
yes to
Sarah (assuming monogamy), going out with Alice is your opportunity cost.