Enjoy oneself, as in
I hope you have a good time at the beach. This idiom, also used as an imperative, dates from 16th-century England, where it was popular until the late 1600s and died out. Samuel Pepys, in a diary entry of March 1, 1666, wrote, "I went and had as good a time as heart could wish." In America it continued to be used, and in the 1800s it reappeared in British speech as well. Also see
hard time;
show one a good time.