This is mainly for kittens. But my male cat enjoys chasing his tail,
so it might work well for him too. Currently, he needs the exercise.

I found a way to give my three kittens all the indoor exercise they
can handle. Basically, it's just extending their tail 6 or 8 inches
with a 1/4 inch thick nylon cord, using packaging tape. It's not very
nice, but I'd rather feel a little guilty than feel angry at them. I
know some people have injured and even killed their pets out of
anger. And these kittens (in addition to three adult cats) have been
driving me crazyer.

To my pleasant surprise... These kittens appear to enjoy their tail
extension. One reason is because it helps a lot with chasing their
own tail. Serious tail chasing action. Sometimes one chases another
while it is being chased. Sometimes they play with their own. I
haven't seen a circle chase yet (with all three kittens). They stop
the action simply by being at rest, and the other cats lose interest.
The most active kitten became so wild the first time, I removed the
thing early so it wouldn't go crazy. But it's just that way, it's
very active. Since then, it's done great.

So... When they start getting into stuff that they're not supposed to
(like all the time), I just whip out the tail extenders and that's
all they do until their next nap. If they lose interest, I will post
saying so.

I'm happy to hear about any potential injury. Most useful would be
personal experience. Apparently getting caught on something is it,
that's why I'm using a breakaway device. And this will happen only
under human supervision.