Get your cats in the habit of a daily playtime session or two (even several times a week seems to work) and you will have playful adult cats. I've been doing this with Harris, Toffee, and Lion since they arrived, and they are still eager to run around and play every day. (Possum, on the other hand, practices only one of his hunting skills with me: the one where he opens an eye to consider the toy that fortuitously landed on his paw. And Wendy thinks I am Going to Kill Her if I send a toy on a string in her direction.)
Harris likes to play so much that almost every morning he goes onto my desk (which I cleaned up dramatically over the weekend) and knocks things around until I get out his laser pointer.
Lion is the sharpest, quickest, most athletic hunter. Toffee has interesting tactics: he chirps at his prey. I wonder if any mouse, bird, or bug has ever been persuaded by cat talk to succumb? But Harris is the most fun to watch because he has no more dignity than a kitten, skidding across the carpet and rolling around with his big feet in the air and his oversized fangs snapping.