Teach him to say “yo” instead of “please”.
“Take me to go potty, yo.”
“Can I have some juice, yo?”
“Read me a book, yo.”
“Ooooh! This is the song that doesn’t end! Yes, it goes on and on, my friend! Some people staaaaaaaaaarted singin’ it not knowin’ what it was–and they’ll continue singin’ it for–ever just because this is the song that doesn’t end! Yes, it goes on and on, my friend…”
When I was 3, my aunts taught me that various foods were supposed to be eaten with my fingers. Like mashed potatoes and oatmeal.
Dye his hair an impermanent but garish color that he will love. (But make sure it’ll wash out quickly and won’t stain his skin or anything.)
Two of my mom’s cousins (brothers) found their mom’s tampon stash and were discovered playing with the awesome “torpedoes” in the bathtub. Food for thought.
::thwack:: back to the corner, and don’t turn around till I say so. 
I hated that damn thing when I was three.
But of course, I wouldn’t want any of you to go wondering what the horrible effects of your suggestions were.
btw, the devil spawn has arrived.
Only the shadow knows…
This is exactly why my brothers don’t babysit my kids.
But just as a thread contribution, you could teach him to say to women “you look fat in that.” Your SIL will just love you.
Mua HAH HAH HAH HAH HAH!!!
Sorry - that just seemed appropriate…
If they’re the kind with a cardboard applicator (and thus no barrier across the end of the tampon), try tucking the “tail” into the little tube, then holding the outer tube and smacking the little tube in hard like you’re “shooting” the torpedo. They’re “one-use” missles this way but may be sufficiently entertaining if you can get some distance on them.
Kiddie jokes and knock-knocks. Give him a book of 'em!
Silly novelty songs – anything from “Witchdoctor” to “The Macarena” to Sir Mix-a-Lot. Weird Al Yankovic could be your invaluable source for this.
Fake dog poop, vomit, rubber vermin, etc.
The most annoying gift that anyone ever gave my brother was a kiddie construction worker’s-type helmet with a rotating emergency light and a piercingly loud siren. He used to run all around the house with that thing on his head going full blast. Just sayin’.
If you have a reasonable expectation that he won’t be putting them in his mouth or feeding them to pets, get him interested in collecting insects and keeping them “temporarily” in specimen containers (you can buy these things at some toy stores or improvise a suitable container, which could be any see-through container with some air holes punched in it).
Why is this evil? Because insects have a way of escaping inside the house, and if they’re not escaping on their own, they’re getting an assist from their keeper, who can’t not play with them.
Similarly, aquariums are evil because kids tend to either overfeed or forget to feed the fishies, and the tank always needs to be cleaned out, and if it isn’t managed well, your nephew’s parents will have to restock the tank regularly and fish out the dead ones. Give him a fish tank – it’s the gift that keeps on giving!
Over the hollidays, I taught my nephews the “Wal Mart game” It’s kinda like patycake, only you say Wal Mart, K mart, Wal mart, K mart, speeding up as you go, then smack the victim in the forehead with the heel of you hand and yell TARGET!
Yes, I love being an Aunt, why do you ask?
Some of these may be a bit dated, but my younger brothers taught them to my sons when the boys were very young:
After belching, look directly at Mom and say, “It’s not bad manners, just good beer!”
Grabbing their crotches while in public and announcing, “Gotta check the package!”
Farting loudly, then proclaiming, “And that’s just the Democrats!” (I have no idea what the hell that meant, only that it was supposed to piss me off. It succeeded.)
The crowning glory: Slug Bug.
Just remember, however, that what goes around comes around. My younger brother now has a baby girl of his own, and she loves to sing. Her two oldest uncles (of which I am the oldest) spent an entire day this past summer teaching her the lyrics of “Louie, Louie.” Gave her daddy a really bad scare!
This is what I was going to say. I taught my friend’s 5yo the interrupting cow joke.* He loved it and told it constantly for months, laughing uproariously each time he told it. And it didn’t bother him one bit if he’d already told you the joke. He’d tell it to you again. And again. And again.
*Knock knock
Who’s there?
Interrupting cow
Interr –
MOO!
Teach him to stare at women’s breasts and say “Giggity”.
Teach him the interrupting starfish joke.
“Knock knock.”
“Who’s there?”
“Interrupting starfish.”
“Interr–”
(plant hand on interrupted person’s face)
My parents said my uncle used to give me my dad’s car keys and tell me to put them in the toilet. I don’t think he ever let me, but my dad wasn’t amused. He later had kids when I was ten or so… When I was 13 I joked with him about it and he suddenly “couldn’t remember”.
Oh… and kid’s drumset. Great gift.
Here I thought this would be all annoying, repetitive songs and lame kid jokes.
That’s repulsive. A lot of these are really gross. It’s sad that there are people who not only encourage this type of behavior but teach it as well.
I’m bookmarking this thread for sure. My nephew is due to be born tomorrow, so I’ll refer back in a few years. I love the interrupting starfish joke. That is utterly fantastic.
When my first nephew was born, I had no experience with children. I had never been around them, had babysat only a handful of times and usually older kids, and I was utterly lost. Shortly after he started walking and talking (not sure how old, maybe 2?), he went through a phase where he was hugging everything. He was driving me nuts, so I casually suggested he hug the vaccuum cleaner standing beside him. He asked me why, and I said “Because vaccuums need hugs too”. He hugged it. Exuberantly. It was at this moment that I realized concretely the absolute power an adult has to shape the mind of a young being. It fully reinforced my decision to never have kids - that kind of power is overwhelming, no? I still worry I altered him for life with that offhand remark - what if he develops some kind of weird housekeeping fetish?
I have gotten a lot of mileage with kids by demonstarting Non- Newtonian Fluids.
All it takes is a box of cornstarch, some water and optionally, a few drops of food colouring.
http://www.scifun.org/homeexpts/lumpyliquids.htm
Their parents now hide the cornstarch like it was poison.
This one is pretty benign, but good fun.
My daughter is two and I freak out her mum by teaching her bizarre words. For example one time I was bathing her we started playing with one of those clear plastic bowls that you put detergent in. I was showing her how if you put it upside down in the water the air stayed inside. We then put a small plastic man inside.
Next time her mum bathed her my daughter said,“Mummy, can I play with my bathyscaph?”
Well, I laughed . . .
At least he won’t lack a sense of humor. Now kindly pick up your organic waste material.
If he’s potty trained, teach him to wear his underwear over his pants.
With a cape.