In the trash, of course. Your toilet should never be used as an "alternative trash disposal unit." It is definitely an environmental issue. I was horrified a couple of years back, when a friend I was visiting dumped an ashtray in the toilet. He explained that he "doesn't like the smell" when he dumps it in his regular trash.
Waste water treatment plants are designed to handle piss, shit, and toilet paper. They are not designed to handle whatever you don't want to throw in a garbage bag.
It's easy to think, "It's not that much of a problem if I do it once in a while..." but when you multiply that times every single person whose waste goes into that system, it is a problem.
Plastic and rubber items, once flushed, get caught up in a screening system, which can get clogged, and must then be cleaned - meaning that portion of the system has to be shut down.
Drinkable water is something most everyone in the USA takes for granted, but the supplies are not unlimited.
Contact your local waste water treatment facility if you doubt that it's a problem.