My Oranda enjoy zucchini and the occasional fruits like pineapple and lemons.
I don't cook the zucc, I just chop in into slivers and mash it with my fingers. The soft center is much easier for them to eat, I don't feed them the skin at all since it's very tough.
You can also cut half-circle slices and make some starter-cut to help them get bites, they'll eat the center out on their own and leave the rind.
I do the same thing with the pineapple, smashing it with my finger before dropping it in. If you give them rings, they'll eat the soft 'meat' of the outside and leave the harder ring in the middle for easy clean-up.
And for lemon (or oranges too), I slice it into slivers and leave it on the peel, make sure to stretch it open so the inside skin is broken (they only like the pulp inside anyway) and they like to pick at the skin, too!
We're new to feeding our fish fresh fruits and veggies and I look forward to see what other people are doing