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.