Stomping on wheat.
…wait, is that ghosts or aliens?
I’m thinking along the lines of OtakuLoki, except that I’d be like the guy that possesses the video game in Strange Brew, eh. You know, that murdered brother, the father of the new owner . . .
Then, I’d possess your goldfish. Make him swim upside down and stuff.
And your refridgerator? I’d possess it, and make it your freezer and your freezer your fridge.
Then, just to totally torque with you, I’d skunk all the beer you bought last night. Just to f*ck with you.
Tripler
You hosehead.
I’d wait until one of my children was on a hot 'n heavy date. Then I’d leap out from under the bed and shriek, “HAVE YOU DONE YOUR HOMEWORK YET?”
Then I’d go to Taco Bell and engrave Jesus’s face on all the tortillas.
You mean that wasn’t the dog? 
I’d go around murdering teenagers. That always seems to be SOP in the movies…who am I to argue?
Anyway, I’d probably want to do something that would leave no doubt that it’s being done by a ghost…like, I dunno, carving messages into solid steel, in front of a camera, or something.
Actually, before I die, I should probably learn morse code, semophore, and American Sign Language. I mean, ghosts seem to have an annoying inability to just out and out speak what’s on their minds.
And if I appear on videotape or photgraphs, I’ll have none of that “rods” or “mysterious aura that looks like a thumbprint on the negative” crap. I’m going to appear as a ****ing skeleton in full robes, carrying my bearded skull under one arm, exposed eyeballs blazing with cold fire. I’m going to make the “Altar Ghost” look like Richard Simmons’ hairdresser.
I’d go to my sister’s house and possess her cats.
I would also spend a significant amount of time scaring people in libraries and bookstores.
I would send them a letter. Only, since I was dead, I’d have someone else write it for me:
A ghost writer.
I plan to spend hundreds of years hanging around old graveyards, and haunting old houses. Upon occasion, I will whisper cryptic sounds into somebodie’s tape recorder, and I will make “cold spots” in haunted houses.
Heck after a few hundred years of this, wouldn’t you get bored?
I’d go back to the thread in GD about ghosts and scientific proof and post “Well HA!”
Seriously (seriously? geez), I’d freak out. Then I’d get my last wishes taken care of (Isn’t the reason for ghosts being around that they’re spirits who can’t find rest / who haven’t had a life desire fulfilled / etc?) so I could join the Great Dopefest In the Sky.
If I were stuck as a ghost, I’d bang on people’s doors until someone paid attention, then get a bunch of scientists to prove me. (So Randi can at least put up or shut up! Watch this thread get hijacked now…)
Kythereia,
who has had too much coffee today
Oh I’m hitting up John Edward for sure.
Just to screw with him.
OK I’m getting a Z is it a Zebra?
Oh yes that’s him.
Ok I’m getting that he played bagpipes.
No
Are your sure? Becasue he is showing me bagpipes? He seems urgent about it?
No. No bagpipes.
Did you play bagpipes or do you know someone who plays bagpipes?
Nope.
Ok now I getting snickering.
Of course if I was a ghost I would be may to the Haunted Mansion in Disneyland and appearing the mirror at the end of the ride to really freak out the regulars.
Oh who am I kidding, I’d be haunting the Playboy Mansion.
Let’s talk… un-dead…
I’d sneak up behind people wearing khakis, howl “booga-booga!”, and see how many wet themselves. Usually before big meetings.