I tried toys with mine - they completely ignored them.
I tried a string of bells tied to the fence (heavy duty, large round christmas jingle bells on a wide leather strap), they each poked it once with their bills, and when no food came out, they walked away, never to return. I tried small dog Kong's stuffed with kitten kibble, figuring if they pushed or tossed it around and kibble fell out, they would love it. Nope. Sat there for a week til the ants found it.
Put tennis balls and floating dog toys in their pool. They refused to go in the pool until I got those weird things out of there.
The only thing they like is FOOD. I drop an entire head of lettuce or cabbage in their pool and they'll go in and tear it apart. I sprinkle dried meal worms in unexpected places so they have to play hide & seek with it. Keeps them busy for a little bit.
Freeze fruits and veggies in a bowl of water, then dump frozen lump on the grass... as it thaws, they chip at it to get to the treats.
Ducks seem to be pretty simple. Food, nap, love.