3. provide reusable water bottles or cups to patrons
It is important that reusable options are available for your patrons to borrow or buy, especially for larger, outdoor events.
Sell reusable water bottles, ask your vendors to sell them, or invite an organisation like Surfrider Foundation along to to have a stall and sell their reusable bottles - this will not only help their cause, but also make it easy for you to achieve your goal.
Offer reusable cups or water bottles with a refundable deposit, which they can choose to then keep as a momentum or return to get their deposit back (there are organisations like Globelet who can provide this service and take back returned cups, they can even brand the cups).