I agree with Sharon Gates that the leap from disposable to reusable cups is where the big eco-benefit happens.
I've used reusable rented cups for events in the past. They work great! We even encourage people to label their cups with masking tape and we provide an area for people to place their cups when not in use so that they can use them repeatedly in the same day. (particularly if the event is big, you will need to have volunteers help attendees and watch the tables) If the rental and washing fees are a problem, you could charge a small rental fee or take donations from the visitors. If you explain you are doing it for the environment, and it is well organized, most people understand and will really learn something from it.
Just another idea.
Human Nature Solutions