Fill up empty seats by selling affordable return trips to unknown/secret destinations. Pick the destinations among those that have low occupancy close to departure time.

Offer pre-filled reusable water bottles and collect them in before landing

Dedicated recycling bins in galley/ head for passengers to use during non-service times

Reusable coffee cups for take away coffee and in-flight.

Separate the salt and pepper from the rest of the table set. Way too often it's just thrown away.

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