Kilmore here's a twist on what you came up with, incorporating my two favorite ideas previously posted. The Vampire and the ball. The way the dryad is actually turned into a malevolent spirit is not by the cutting down of her oak tree. Near the oak tree was the resting place of a Vampire. This Vampire had been an evil Ranger in life (using their skills to hunt down other humans for hire) and continued the malevolent pratice in death. When the loggers came for the oak, the Vampire sensed an opportunity so it did not slay them. It directed to the Dryad's tree, a tree so large that it could not be cut down in a single day. The Vampire came to the dryad the evening after the loggers began cutting down the tree offering it the chance to avenge not only it's own tree but all the other trees murdered by the loggers greedy masters. The naive Dryad in it's turmoiled state accepted the Ranger's offer and was slain, only to rise three days later as a Vampire Dryad. The owners of the boat (and the loggers masters) live on the destination island, the Dryad has been murdering people on the ship to eat as food, and attends the Masquerade ball thrown by the Captain as... a Dryad, which seems like an excellent costume, one that wins it first prize. First Prize is reimbursement of a passenger's fare (good reason for PC's to participate) however the Dryad has not bought a ticket, so it is revealed as a stowaway... take the action wherever the players go from there... it could lead to several things, including any tree-hugger PC's avenging the death of the tree AND eventually hunting down the Vampire Ranger (if they get to chat with the Dryad before defeating it. Add some sort of threat to the well-being of the boat and you could force the PC's and the Dryad to work together, after all neither side wants the boat to sink before they reach the island....
