Make the app as lightweight as possible. Remove images, animations and leave only the bare necessities. This saves data transfer and storage - which in turn saves energy.

Option to make a plant a tree donation. By having a partnership with a existing tree planting project.

Do not carry tax-free goods on flights. Use as online service and if necessary in partnership with partners allowing pick-up at airports.

Instead of making passengers pay for biofuel, issue carbon credits for the emissions avoided by using biofuels instead of conventional aviation fuel.

My personal CO2 emissions should be listed on my boarding pass in the app for each flight.

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!