Every hot drink on the square comes in this year’s deposit mug (sample). You pay $3 on top of your first drink; the hut gives you a token with the mug.
Bring the mug back to any drinks hut, not only the one you bought from, and you get the $3 back. Or keep it: the design changes every year and some visitors have a shelf of them.
The mugs are washed in the Corn Exchange kitchen and go back out the next day. Last season that saved about 60,000 paper cups (sample).