Getting more passengers on a bigger modern planes on certain routes will lower the emitted carbon dioxide per passenger as well as optimize the use of flight crews.

Sponsor projects that can help build renewable energy sources, like wind mills, solar panels. Additionaly work with selected airports on becoming carbon neutral

Invite all travellers to bring their own coffee cup.

Include meal information as early as possible in-app.
This is good for people with allergies, so they abstain from it.

Use powder soap instead of liquid soap in all lavatories, powder soap is much lighter thus reducing wieght.

Earn EuroBonus points when recycling. Collaborate with apps such as Bower and Repeat in Sweden. They offer cash rewards for recycling. Why not reward points instead!