Water.
Or sprite if you want it to look like you may be drinking something. As a bartender, I used to get asked this fairly often. Have a juice, no one will ever know it doesnt have alcohol in it and have them put it in an alcoholic drink cup versus a soda cup.
The most recent bike night I went to the owner of the joint was offering free margaritas to the riders to have it there. Seriously??!! It was nice hanging with folks and meeting new faces, but I just cant get into the whole drink and ride thing, even if its just one.