During Ghosts Ahoy, you must beat Robin at a game of Rune-Draw so that he will sign an Oak shieldbow for you. You can then give the Signed oak shieldbow to Ak-Haranu who is a fan of Robin.
After completing the Medium Morytania Task Set you can claim the Morytania legs 2 from Robin. Additionally, Robin provides the ability to trade him 13 Bones in exchange for 13 pots of Bonemeal and Buckets of slime per day. After completing the Hard tasks, this is increases to 26 and after completing the Elite tasks it increases to 39. Robin will accept noted bones and provides the bonemeal and slime in noted format. To save time make sure to use the auto-convert option.