I luckily have a book about having poultry in the backyard so I found out that it likes:

•  flour-all kinds,
• grains -soaked,
• a few seeds and 
• nettles chopped finely
• even a bit of coffee marc to boost their health

She eats only near water. The first time she even had to sit in the water to start eating. She would just nibble on the sides of the bowl, not eat inside otherwise. Funny!

She loved it! We were happy witnesses of her meal. She would take some soup in her beak, dip it in the water then swallow it, hastily.

The children also gave out lots of soaked home-bread bits, which the ducks steadily enjoyed of course. They even ate out of the hand.