Compost pile or mulched area.
They can scratch through it to get bugs.
I know there are limits on how much material you have available to compost, but it's often possible to get a truckload of wood shavings for free--just spread them several inches deep somewhere (maybe a shady place where grass doesn't grow well.) Within a few weeks or months, there will be plenty of bugs in there, and the chickens will spend a lot of time scratching for them and eating them. And it doesn't take much effort on your part--once you've got the chips in place, it's all set for the next year or so.
Chickens do like to eat grass, but I think they get more food value from the bugs in compost or mulch than from the same area of grass. Of course, some of each is better than all one or the other.
I would also offer the chickens any food scraps you have. For convenience, just dump it on the compost pile each day and let the chickens pick through it. When they have enough choices, they are quite good at avoiding things that are bad for them. And you can include things like meat, fish, and grease (unlike a normal compost pile), because the chickens will eat those up before they have a chance to attract pests.
The scraps from a household might be too much for a flock of two or three chickens, and make their diet unbalanced, but that is not much of a concern when you have 60 chickens. No one chicken will get enough to be a big problem.