Dog chew toys can come in handy even with ducks. So you can get them rubber bones, knotted ropes, or any toy meant for chewing. Just ensure it’s not something they might swallow and choke on.
As they run around and nudge the bell, it makes noise. The noise from the bell intrigues them, and the ducks could go hours trying to spot where the sound is coming from.
Ducks love swimming. They are waterfowls, after all. Provide your duck water to swim and splash in. They’ll love it. You can get them a kiddie pool if you don’t already have one.