Ducks aren't really toy-lovers in general. But if you want to entertain them, the best thing is to give them entertaining food. For instance, they love to chase crickets and minnows (both of which you can buy cheap at a pet store). Greens chopped up in the water will keep them busy dabbling for hours.
If you can make them a safe outdoor enclosure to run around in the grass inside, they will entertain themselves chasing bugs, digging holes in the ground.
Of course, water is the ultimate entertainment for a duck, but you have to be careful with ducklings because they can get chilled very quickly when they're damp, and they can't float forever like adult ducks can.
What kind of ducks do you have? How old are they?