Giving an incentive and getting more passengers to travel light should reduce aircraft loading and fuel consumption. Alternatively allow more cargo.

For meals, have condiments provided by request, and not automatically given to passengers. So many food items are unnecessarily thrown away.

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

Arrange crew accommodations closer to airports to reduce transportation emission

Invite all travellers to bring their own coffee cup.

Use larger packages for the most consumed drinks, instead of handing out endless amounts of cans (less garbage, less weight on board) .