I agree with the straw bales - hours of fun.
I've also tried hanging up DVDs as pecking objects (actually from the cover of the Garderner's World magazine, with Monty Don on) and a light weight football for them to push around.
When I put them in they all look at me as if I was insane, and they've steadfastly ignored them ever since...
If you tie up greens they'll happily stretch and peck, or jump up to them if they're higher. I heard that it was good exercise, but I worried about their joints when they landed.
The favourite activities of mine are Cauliflower Pecking & Chicken Rugby. Put a hole through the middle of the cauli with an apple corer, hang it up and watch them demolish it.
For Chicken Rugby, throw some grapes in. Mine got mad for them, grabbing one then running off, before being chased by the others, who try to grab it. Hence, Chicken Rugby. Always makes me smile that they'll focus on one grape & scuffle over it, ignoring the other 5 that have been thrown in. Until, that is, the 'lower level' chooks sneak in and eat the surplus.
I should stress that they don't hurt each other, just push to get the grapes. I wouldn't do it if I thought they'd get hurt.
The children love particularly love putting the grapes in. Hours of fun.