I feed my ducks food grade diatomaceous earth as a parasite killing suplemint.
I have also fed them:
bananas
apples
cabbage (they didn't like it)
carrots
celery (it's sooooooo funny to watch them devour it!)
lettuce
spinach
oranges
leeks
grapefruit
pears
mealworms
and some other stuff that I can't remember
My ducks LOVE frozen green peas (in warm weather, I leave them froze; in cold weather I warm them up). Are you cooking the vegetables listed above or serving them raw? Mine only seem to eat peas as treats. Of course the worms and other bugs they find in the yard too.