I liked the idea of the tunnels, but it just isn't practical here. This is what we do instead:
They get a new area each week and they are protected from land and aerial predators. We call it the snake fence because it's long and skinny and has lots of twists and turns.
If you decide to NOT let them roam, check out my post on chicken entertainment. It will go a long ways towards preventing bad behaviors.
Oh yeah, that's an even better idea! That way you can change the direction every week. That's a great idea.