I also tried the watermelon and all they did was lick it... no bites?!?! Would it be a bad idea to place the fruit in their water dishes in water.. maybe they could get some nutrients from drinking?? Btw all 13 of my Leos drink water from their dish, I have heard others say that leopard geckos will NOT drink from a dish (I have read it also) however I see it every day and have seen it with other leos I have owned in the past.
Mine will close his eyes and lick at watermelon for a few secs, then will bite it. He seemed to like it and has taken it multiple times. Idk if he likes the sugar or simply eats it for the water.
So I know geckos are very different from dogs and birds but I just thought I would throw something out here. Avocado, onions, and grapes are all VERY poisonous to dogs, as well as chocolate. Avocados and onions are also something you are never supposed to feed birds, and they naturally eat quite a bit of fruit and veggies. All Im saying is you might want to research a little more just to be safe. Just my opinion.