Complimentary Arlanda Express ticket when booking a flight to/from Stockholm

Voluntary fee of 5 cent/mile that goes to research and developement all electrical passenger airplanes. The aircompany gives another 5 cent per same miles.

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

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.

Solar panels to generate energy

Use travellers feces from flights as a bioproduct. As fuel or to something else useful.