They are protected from many contagious diseases, and injuries from other cats, dogs, and cars. But indoor cats have more behavior problems like urinating or defecating where you don't want them to, excessive grooming, or being aggressive towards people or other cats. Anxiety, dissatisfaction, frustration, and boredom may be the underlying problem. There are some things you can do to enrich your cat's environment that may help to avoid some of these problems. You may be able to find locations in your house or apartment for perches, shelves, and scratching posts.
Cats like to climb, and explore things.
Your cat may enjoy playing on a series of shelves, perhaps padded with carpet remnants, and at varying heights, close enough together so a cat can jump from one to another. Cats like to climb up to a high place, where they can sit and unobtrusively keep an eye on things. Maybe they're pretending to be a leopard, waiting in a tree to ambush a deer. Cat's love to explore and investigate new things in their environment. When we buy something new, that comes in a big box, our cats love to jump into the box and hide, and peek out.