I got a motion activated sprinkler last year, but it had too many blind spots, and sometimes the cats would slowly walk right past it without triggering it. It was very satisfying to watch them flee when it did work, though.
I planted a new bed with wildflowers, Passiflora incarnata, and Morus nigra seedlings just yesterday, and already today the cats have dug up about a quarter of it and left their nasty nuggets of filth. There are at least 4 cats I see regularly in my yard, including one that's feral, so there's not much I can do other than fencing, sprinklers, and wishful thoughts.