Airships might be nice for tourism, but for carbon neutral I might take an ethanol or hydrogen powered plane to save time. No reason why you can't run a plane off biodiesel, but I'd be worried about partical pollution and how the stuff reacts to low temperatures. Hydrogen powered jets might need big fuel pods, but the stuff is light. But it might be easiest to use kerosine from oil as we do now and then remove the carbon from the atmosphere with plants, by abusing plankton or other methods.

But good luck to the giant flying turtle company. They might need it.