Most canned peas have too much salt. I just get a cheap bag of frozen peas and thaw them out in the fridge. I also have an arrangement with the local mini-mart where they call me when they have lettuce that has gotten too wilted to put in their sandwiches.
I didn't realize it, but I was always humming around them- stuff like "Amazing Grace" and other simple tunes. Now they are a year old, and if I start humming around them they come close (not too close!) and settle down in the grass and get very still and calm. Khaki Campbells are among the more skittish of ducks, so normally mine won't come within 4 feet of me, unless I'm humming.
Another treat my ducklings absolutely adored, especially at bath time, was Cheerios. When they were really young, they had to wait for the Cheerios to soak up some water and get soft, but as they got older they would zoom in on them as soon as they hit the water. After they were about a month old, I occasionally bought a batch of feeder guppies from the pet store and threw those into the tub. But I'll warn you- if you have problems 'holding it' when you are laughing really hard, be prepared!