Issue extra bonus and level points to passengers who decide opt out of their included meal ahead of a flight.

Only fly when tailwind

For hand wash tissues, hand out from a larger container instead of singles wrapped in plastic.

Introduce flying quotas, personal and for companies

Have a separate bin to put old baggage tags and recycle them. Not together with ordinary waste.

Use travellers feces from flights as a bioproduct. As fuel or to something else useful.