In M class on Intercontinental flights reduce the usage of plastics, ask the passenger to keep their glass for the entire flight before collecting it for recycling.

Encouraging standardized suitcase measurements and/or using AI to plan and stack luggage for saving space and even out the plane in regards to weight.

Solar panels to generate energy

If you reduce the allowed weight for luggage from 23 kg to 15 kg or so, people might start to think if they really need all that stuff with them.

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

Charging mobile phones or other equipment from sunlight in the sky. «Day flights are always sunny»