I'm not a fan of those hollow cement tubes that are placed in many carnivore enclosures. There is a Canadian zoo called Discovery Wildlife Park that utilizes the idea to such an extent that the concrete detracts from the surroundings. I find the tubes so completely unnatural that they really spoil what could otherwise be a decent exhibit.
@ThylacineAlive Yes, absolutely! You could easily open up a whole can of worms in regards to a debate over zoo enclosure aesthetics. Why not just have massive, thick, chain-link fences and a pile of ugly cement blocks in all directions? If the animals are "happy" in the space then does it matter to visitors? Of course it matters.