Incentivize sustainable behavior, e.g. a small reward for bringing your own coffee cup. This can also be sold by SAS in the Eurobonus store or in flight.

Granting a number of KG per year per passenger for onboard / checked-in luggage

Give/sell digital bag tags to frequent flyers

Let's start by using SAF at least for domestic flights.

Include 1 free unit of biofuel per booked flight, for passengers that have reached Conscious Traveller status.

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.