have (oneself) (something)
To enjoy or indulge in something. I'm going to sit down in the shade and have myself a cold glass of lemonade.
have oneself something
to get, have, or take something. I'll have myself some of that coconut cream pie, if you don't mind. You just go and have yourself a nice, long nap.
Enjoy something, as in Be sure to have yourself a good nap, or They were having themselves a great time at the fair. The oneself in this colloquial expression adds emphasis to the verb have.