Come in early (before everyone is wasted), stay for about half an hour.
If someone ask you if you want a drink say:
yes, a diet coke would be great. Thank you (or whatever non alcoholic drink you like)
Leave it at that. I betcha anything that no one asks you why you don't drink.
It's not just alcoholics who don't drunk you know, diabetics, people in some religions like Muslims, Buddhists or Mormons, people into fitness and then some people just don't like it.
When you make your early exit, if someone ask you why you are leaving early just say:
I have to get up early tomorrow.
No need for details.