I think they would love the rock. They love having something to 'get up on'....especially the rooster.
We left the brooder in the run when we moved the chickens out there.....why? I have no idea and I questioned my husband many times over the past year as to the reason. Now I have eggs in the incubator and a small brooder to put them in....but I want the big one. The run has a wire top on it that will have to be lifted off while we move it out....so there has been a delay in getting it done.
It was made from the old liner from a pick up bed....we recycle out here a lot. He put two 2x10's on the end to close it off and built a frame with chicken wire in it for the top. We cut out a square to put in food and water and I had something I could use to close that off. With 29 chicks it worked great! In fact they stayed out there longer than we meant for them to do and it just seemed easier to drag the whole thing out there than to try and catch them. The neat thing about this liner is it had been in the woods for a long time and had decayed leaves and dirt in the bottom and the chicks, who were feathered out by then went nuts digging and scratching in it.
But the point of telling you that is so you would have some idea of the height of this thing. Above knee high. They all get up on it and walk around and take in the views. When we take it out next month not only will they go crazy for the bugs under it but that is new scratching ground for them. I will have to replace it with something for them to get up on!
The other run has a 'chicken cabana' in it. I made it from a crate that was given to me and put it on bricks to keep it off the dirt and possible decay. It is a pallet with sides. I put a piece of tin from the barn on top for a roof and a roost inside for just in case. The pullets used to get inside and hang out. The rooster gets on top every morning and crows from there. Often the girls are on top with him.
So, yep on the rock....it will entertain them.