We built an outdoor cat tree for Charlie and planted grasses so he can hide. He also has his undercover mouse outdoors on nice days, ping pong balls and purry curlies round the garden, bubbles and tunnels. He has a cat house which he NEVER uses and I am putting up some shelves along the wall for him to climb on. He also has a water feature which I thought he would love but he studiously ignores. I hide treats round the garden (obviously on nice days) and we play hunt with his Da Mouse.
He also, of course, likes to join in the gardening and watering.