Encourage food stalls, merchandise stalls, exhibitors and other vendors to be as sustainable as possible. This can be the products they are selling, the physical set up of their exhibit/stall, or the way they are operating. Put signage out to recognise the winners and offer them discounts or free pitches at future events.
The trader in the image to the right was the ultimate! She harvested discarded tents from Reading Festival and made them into rain ponchos and bags to sell at Glastonbury Festival.