Cats love to climb, reach higher ground, and use the scratching pad to file down their nails. This is just what they do.  You might not get the impression that cats are active, but they are.  Since homes generally lack trees, the Cat Library should make for a nice solution.

The Cat-Library shelving system is modular so you can move the shelves around to keep your cat entertained and it even looks like there is a built-in seat.