Make something fun for your cat and have fun doing it. My husband and kids made this box tower playhouse for our then young cat. It has many levels, two observation decks and plenty of interior places to explore.
Gather boxes. Cut holes. Tape together. I could give you step by step instructions for making this box tower but that would take away half the fun of it. Be creative with your design.
Our box tower has holes at the base for the cat to enter. The stacked boxes have holes in the top of one box and through the bottom of the box above so our cat could climb higher. Windows were added. The most important part is the observation deck. Cats like to have a place to perch up high.