Collect recyclables and make art grow during your event. Use chicken wire and metal to create a frame and then fill it with cans. The photo on the right is at Sountbound Festival in Western Australia.
You can also flatten the cans and stick them on the outside of the structure if you're wanting to make a dragon or fish! This is what they did at Electric Picnic in Ireland.
Use plastic milk bottles to make an elephant like that did at Glastonbury Festival in the UK.