We've started to have all of our mixers at a public venue...if the people who are 21 and up want to drink, they can, provided they bring their own money for their drinks. So far this semester we've gone bowling, out to a local bar and grill, and this coming week we're going out to dinner with one of the fraternities.
Dry events can sometimes be more fun than alcoholic ones...you don't need the alcohol to have a good time, and besides, people may remember the event the next day