I kind of chop up my lettuce for them, too. I don't think they can "chew" but smaller pieces go right in. I also grate carrots for them, and they seem to like it.
Someone said cabbage, too, but mine don't seem to like it
We just put our 3 day old ducklings in the tub. We kept it shallow at first and threw in peas. They had a blast. We then added more water and they had their first swim! They were a real treat to watch.