LOL....my cat Jub sounds like this. But, no, trust me, he's NOT retarded. I don't think it's even possible for cats to be retarded. My cat does ALL these things and more. I can't even begin to list the kinda things he does lol. But, don't worry about your cat being retarded, the vet is right. If he's physically healthy, you have NOTHING to worry about. If he's an indoor cat only, he might be meowing cause he's in heat. Or, he wants to go outside to play with the other cats he sees (If he sits and watches the outside world from a window) Him playin in the litterbox is just that....playing...maybe a sense of wonder. Same with the shower and the toilet....they've gotta learn for themselves on what the boundaries are. Some cats even like water until they're forced into it...then, they'll hate it.

My cat walks on my head while i'm sleeping...he'll meow and jump up and down while i'm sleeping. It doesn't help having 3 other cats in play mode while i'm sleeping, especially since they're all about 3-4 years younger than Jub.