Here is a list of things I have offered my ducks and their reactions...
Cabbage: Went nuts for it
Bok Choy: Same as cabbage
Celery tops and leaves: Ate some of it, but I dumped a lot of it too
Radish greens: Wouldn't touch them
Kale: LOVE it
Spinach: Also love this
Red Peppers: Ok, this one is funny. I had some end pieces so I chopped it up fine and floated it in the water like I do their greens. They saw me put stuff in their play pool and came running. Then they saw the color was different and came to a DEAD STOP. They stared at the pepper pieces for a good 20 minutes before Janet (my khaki girl) got brave and tasted the peppers. She liked it, so all the other ducks decided it must be good and helped her eat them all. Now they are used to them and go after the peppers almost as much as they gobble up the cabbage.
Cat Food: The like some of the dry food I have given them, but wont touch the canned.
Jicama Peels: Only Janet likes these.