I guess I wasn’t thinking interspecies transformation, just more subtle stuff, like changing their ethnicity or sex.
I would install a brain in George Bush. He would then have a hearty chuckle over what an idiot he used to be.
Two words:
Transporter Nakedness!
Bosda, can I interest you in a pseudo-death ray? If anyone will build it and drive around the capital, you will.
Rio by Duran Duran?
Turning a prehistoric monster loose in the middle of a city is always good for a laugh.
Also, you can have lots of fun with pieces of stable neutronium. For example, putting a 500-lb sheet of it in a junk mail return envelope, so they have to pay the postage on it.
I send Joseph McCarthy back to Salem, and then whenever he walked by me in the street, I’d drop to the ground and pretend to have convulsions.
go to a party and make all the molecules in the hostess’s undergarments simultaneously move a foot to the right, froody
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Emperor Wang’s sex ray from Flesh Gordon.
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Republican National Convention.
No assembly required.
Definitely torture politicians I consider unethical. That Jim Carey movie stole the funniest one (changing what they say) but also fun would be to use my computer to make high quality animated pornos but photo quality and indistinguishable from the real thing. Then release them to the file sharing networks. Fiddle withtheir minds so that they hear God saying the things I want them to hear.
Similar to the gas in the OP, I want to crash the worlds silver, gold, platinum, diamond (especially diamond) markets. Just need a machine that can make a couple of tons of the stuff an hour.
Crash the ratings for shows I don’t like and the shows I do like make their ratings really good.
If I had a time machine, I would borrow the 42-reel version of Eric Von Stroheim’s Greed from 1924, digitally edit it to include an extended scene in which McTeague takes advantage of Trina while she’s in the chair and under the influence of nitrous oxide, (which would make Blue Velvet seem like family fare,) print it to period film, and arrange for the aged celluloid to be found in Al Capone’s secret vault in 1986. I’d also make a crude love doll in the likeness of J. Edgar Hoover using 1920s technology and leave it in the corner on a bare mattress.
Maybe replace all the amphetamines Adolph Hitler took with MDMA.
Show up during a battle of the Revolutionary War with a couple of fully loaded Apache helicopters or Abrams tanks. While I’m there, I’ll make all kinds of recordings (both video and audio) of the Founding Fathers doing things which would shock people who seem to believe that they were saints. (I’d also slip Ben Franklin copies of modern engineering textbooks, the Kama Sutra, and the complete Joy of Sex series. :D)
Travel back in time and swap the baby of some nice, Jewish family with that of the infant Adolf Hitler.
Show up in the time of Christ with all kinds of nifty technological gadgets and challenge Jesus to a “Miracle Off.”
Convince John Wilkes Booth that the paintball gun I’m holding is a much better weapon to shoot Lincoln with than his gun. (And that while he’s going up the stairs, he needs to hum the “Mentos” theme, then, just before he shoots Abe, he needs to say, “Hasta, la vista, Abey.”)
Carve the Marx brothers images on Mt Rushmore about 2000 years before Columbus is born.
Paint a giant smiley face on the far side of the Moon.
Place a large, mysterious monolith near the Apollo 17 landing site.
Place giant, rusty tripods close to where the Viking landers will touch down.
Oooh, excellent!
How about recreating the “The Day the Earth Stood Still.” Fun for me and might also do some good.
Anything from here.
•Get some of that theoretical gas that causes rabid, homosexual behavior in whoever inhales it, and drop it…God, anywhere would be good. If I had a time machine, I’d drop it on Woodstock, right before all the camera crews showed up. Or the Sermon on the Mount.
•Create/steal Mechagodzilla (or the equivilant), and use him to attack Israel/Palestine. (If that doesn’t create “solidarity through a common enemy,” I don’t know what the hell would…hmm, maybe two Mechagodzillas?)
•Dump 2-4-5 Trioxin on the mummies of Mao, Ho Chi Minh, and/or Lenin. Maybe Otzi the Iceman, too.
•Hire an alien with a terminal disease to crash-land a small ship in front of the U.N., and croak out, in English “came to…warn you all…we can’t hold IT back for long…you must—must flee this place…IT’S coming here…to this world…It’s coming…!” before dying.
•Use a weather machine to cause a major blizzard to hit a major enviromental conferance, or Al Gore rally. (Obviously, only if it’s somewhere that shouldn’t possibly get snow at the time, if ever at all.)
•Use a Genesis Device on Pluto, turning it into a lush paradise kept warm by geothermal activity and a thick but breathable atmosphere. Who’s not a planet now, International Bitches Union? 
•Use a mind-control device to cause several Gombe chimps to “discover” how to make fire.
You could have some fun combining these two…
“Verily it is written, an eye for an eye and a tooth for a tooth; but I say unto you, why stop there?”
– Adolf the Christ
The best idea hit me last night. Go back in time to 9/11 and at the last second before the planes hit, teleport them to the Moon. This will scare the crap out of everybody, because there will be ample witnesses that the planes just disappeared, Osama & Co. will have no idea what happened, other than the planes are gone, the US gov’t won’t know what happened, other than the planes were hijacked and disappeared, but naturally, will want to put the best face on everything, so they’ll issue vaguely worded denials, which will make everyone on the planet think that we’ve got some kind of super weapon we’re not talking about. Then, whenever someone gets back to the Moon, they’ll find the planes and wonder how the hell they got there?
Regrettably, you would be a day late.
I hope you’ll also teleport the innocent passengers to a friendly location with air, such as Australia.
Better yet…teleport the towers away, at the last moment. I’m thinking you could have them appear inside Mecca (or maybe on a coupla anti-grav platforms over Mecca), then return them to New York after the terrorists were neutralized.
That outta cross a few wires.
OK, so I pop back to the present, pit myself for a dumb mistake, and then pop back on the right day 