People travelling independently to Symi might be interested in the latest special offer from Dodekanisos Seaways. They are offering up to 50% discount on the return leg of return tickets.
This sounded too good to be true so I've just tried it. Lo and behold, it works! The online booking system has given me the usual 10% discount with the Bonusways card on the outward crossing and a full 50% on the return leg, even though the return date is within the 20% discount period. I assume the level of discount is determined by the date of the outward trip rather than the date on which the discounted crossing takes place. I don't know if that's a mistake but I'm not complaining!