When the crew collects trash before landing, why not make separate rounds for collecting, cans, plastic cups, paper, etc instead of collecting all trash in the same bin.

Let it be easy to take steps when buying bio-fuel.

Earn EuroBonus points for cancellation of non-refundable tickets at least x hours/days before departure, so SAS can sell the seat to someone else

Bio fuel / green fuel as default, and having to pay more as a passenger if you want to fly on fossil based airplane fuel.

Invite all travellers to bring their own coffee cup.

Stand up seats for shorter flights.