I just kinda try to play up not feeling well and hope that works.
I only do that if I really don't feel all there in the first place, however. I don't tend to try to make up an excuse if there's not a hook for one already present in the first place.
Though I guess I did get out of having to come back into work once - it was not scheduled, it was a "can you fill in for this person on very short notice" deal - by claiming "I'm busy right now." Yeah, busy playing a game, perhaps.
(Although because of that and because of some other wrongful assumptions attributed to me at the time, I got quite the earful about that the next day over it, which surprised me since it was the first and only time I had done that. So uhhh, maybe that's definitely not the way to go, haha.)
I dunno, all mine tend to be vague. Not feeling well, busy with things, have something else planned, etc.