Moving the cat tree or kitty condo near a window or glass door will give him an “outside” interest. And the view doesn’t have to be that of animal or bird activity; cats enjoy watching people and cars, too. There are also lots of battery-operated toys that can be pre-programmed to engage your cat at regular intervals throughout the day. Some cats even learn how to switch on these games themselves!
If your cat is home alone, there are also special cat videos that can be set to play on a loop throughout the day. Also, create a treasure hunt for him by hiding treats around the home in his favorite places so that he can “hunt” them out.