If they do yoga classes on the ship, I'm pretty sure there is no additional charge.
I just found out that our airfare went down for our flight out, I love JetBlue because they are giving me a credit for the difference to use on a future flight! Hooray!
Fish Extender gifts can really be whatever you'd like. As lucky said, magnets, Disney items, treats, glow bracelets, etc. In the past we've done glow bracelets and homemade mini scrapbooks. They can be 1 gift per cabin or 1 gift per person. I'll get a list together closer to the cruise with the cabins and ages of people participating. Some people gear the gifts to the different ages, some don't, and whatever goes. I loved it when we first participated and DD was just 2 and there were plenty of gifts that were geared to her age (Disney washcloths, little toys that weren't choking hazards, bathtime books and squirty toys, etc.).