Keeping your cat active while indoors will help keep kitty at good mental health. In the wild a cat would spend most of its time hunting prey so its healthy to keep your cat busy while indoors and to avoid boredom and obesity.
   Get your cat a scratching post so it can stretch and play. Train them to perform tricks and reward them with treats. Cats also love vertical space or places on higher levels that they can perch on. Get an aquarium for kitty to watch fish swim or perch him/her near a window so he/she can observe the outside.
   The idea is to keep your cat's mind occupied and to entertain curiosity to maintain mental health.