Most of the things mentioned here already, lol.

+ since I live in a small apartment with 5 cats now, I've put up "catwalks" on the walls so the furbabies can literally run all over the walls in different levels and jump from one to another. I also made 2, 8 feet long (from floor to ceiling), scratching poles they can play on (they're in connection with the catwalks on one of the walls).