We have 12-14 week old ducks. We've had them nearly a month.
One of the biggest hits we have had with our ducks is watermelon on the rind. Ours didn't like being handfed watermelon (everything else, yes). So, one day after the family had watermelon, I took out the rind that had about 1/2 inch of red still on it. They went crazy. It was hilarious. They were so excited that the youngest, Diana, ended up with pink feathers! They cleaned it out and slurped up the juice. When we bring out watermelon, they chase us to our chairs.
Our ducks also love grapes. I tend to halve them and hand feed. They also love tomatoes. Grape and cherry tomatoes, sliced up make a good treat, and each of our ducks prefer different colors. Several of ours love the yellow grape tomatoes best. One of the things our ducks love to do is hunt for the grape halves in their pools. I only put them in there in the morning when the water is fresh, but they love searching for the bits on the bottom.
Romaine lettuce! We buy a head of it at our HEB. Nice, large deep green leaves that they just love. It is probably the easiest thing to hand feed since the pieces can be larger. We probably go through a head of romaine every three days.
We tried bananas, but our ducks were not thrilled with them at all. We tried boiled, saltless potatoes. They turned their noses up at that.