How did this turn into a cat killing argument (again!)? I hate them, but would not use antifreeze, only due to its non-selective nature. Even small children go for the sweet flavor and pretty color. Not a nice thought. One way, which is slightly more selective as far as preventing humans, is a 220 Conibear trap in a 5 gallon bucket. Quiet, very effective. Legality varies, so check. Any trapper can show you how to make this "bucket cubby" set. BTW, State Fish and Game Departments in the East consider feral housecats to be THE #1 killer of small game and songbirds. They kill more than hunters, coyotes, foxes, hawks, owls, and all other "normal" predators. There is damned good reason to kill them. If they are someone's pet, fine, but if they are off the porch, they are wild and a fair target, IMO. If you don't like that, don't kill them, but LEAVE ME THE HELL ALONE. As far as saying I shouldn't own firearms due to that attitude, I would ask that you say it to my face . . .
To answer the original ? I like reactive targets too, especially if a newbie is along. Any kind of water jug is great, as are such things mentioned above like clay pigeons, cookies, alka-seltzer, etc. Human silhouette targets can readily be made from suitably-sized cardboard and are fun.