With Monzo in the press today with its drive towards reducing the amount of paper receipts.
An idea to advertise the fact that Monzo is an environmentally aware bank.
Maybe as a reward or for a small cost Monzo could produce reusable coffee cups with the Monzo logo on.
Then when non Monzo users see these it would be a form of latent advertising that Monzo are an environmentally aware bank. Also helping with the drive to reduce the usage of disposable coffee cups.