Chickens: any kind of tub that has dirt for dust baths, hang cabbage or lettuce on a string just out of reach, mirrors, old cd's dangling where they can peck at them, make them a swing with a thick tree branch and some rope, any kind of container that you can stuff with treats that will either slowly fall out as they shove it around (like a peanut butter jar with dime size holes in it filled with sunflower seeds), also they love to perch so various perches of different heights, chickens love to hang out on tree stumps so a few different size firewood rounds would work perfect, a compost pile to scratch in, tree branches leaned up in a corner (check online to find out which kinds are toxic to chickens) for them to peck at and hide in, grasshoppers or crickets turned loose in their run, basically anything that promotes scratching and pecking is great. Careful of the treats though. Try to use healthier ones so you can use them more often
Rabbits: sticks to chew on, tunnels to hide in and play peekaboo with each other, anything wooden and nontoxic to rabbits that they can climb on and chew on is always a hit, a large but shallow tub filled with dirt to dig in, treats dangling on string, mine like a big pile of grasshay in the middle of the floor that they tunnel in and kick around and munch on, and even just a cardboard box will keep them entertained for hours!
I don't have ducks yet so I can't help with that one.
