George is now pretty much free range, so I don't have a cage, but I do put a baby gate across the door of one room when I go out and at night to prevent him from going everywhere in the house.
When he first arrived, he had a big indoor cage with a pen attached. All the things you have already said, plus plenty of toys and tunnels.
My husband boxed in all wires so that was the most major thing to do, but didn't take him long.
I have found those utility cubes from Argos have been great to create barriers in certain place you don't want them to go, I have them in a V shape at each end of the sofa to stop him going behind there.
The pen I bought can be shaped into lots of different ways, so it can be a barrier fence or a complete pen, this was really usefull to stop him getting places he shouldn't when he was smaller, now he's bigger, older and neutered it's not such an issue as he's really good.
Another thing I found great were the seagrass mats from the 'hay experts'
I used to put them in the corners of the room so if he did fancy a chew or a dig then it was them and not the carpet. If he used to scratch at the carpet, I'd clap my hands say No and give him a mat to chew, seemed to do the trick.
I now just use the utility cubes made into a corner shape, to hang his water bottle on whch is stood on a big tray in a corner, he has his bowl of food a hay rack a catbed with towel and fleece blanket in, his litter tray and plenty of toys and upturned wicer baskets etc to jump on and he seems happy as larry.