Not a very realistic scenario, but anyways:

A man was at the New York airport, and he desperately needed a flight to Thailand. Unable to find a quick route there by any commercial means, he noticed a man outside with all the other planes, about to climb into a fancy-looking, sleek jet plane. He found his way outside, and ran up to the pilot, panting. "Do you think I could get a ride to-". The pilot interrupted him, "Sure! It's right on my way. You can tell me while we're flying. Hop in." How could the pilot take him on without being yet told where his destination was?

(there is a minor descrepancy with the answer, but if you know the discrepancy, than you know the answer I'm looking for, so I guess I'm being redundant)

Pilot's going everywhere in the whole world?

Not THAT unrealistic, SK.

Although that would tie it up nicely.

Edited question.

Luggage tag had his destination on it?

Nope, sorry.

(tiny celebration for not being solved immediately)

*pulls up Google Maps*

Maybe the pilot was going to Singapore. Which ever way he flies (easterly or westerly) Thailand is on the way?

Nope. The pilot knew that it was on the way without being told where the man was going.

Was he wearing anything which would tip the pilot off?

Nope.

He was flying to the exact other side of the world, so he could go by any route to get there/

Yep. Unless you're talking about the slightly bulged earth where it's wider around the equator than it is pole to pole, he could fly in any direction. Also works with a circumnavigational flight. +1 Bard.

I guess that works if he can pick his route, but don't planes normally have a pre-ordained flight path? 

@SK - If you're a private pilot, you can set up your own flight plan.  Whether or not the FAA makes any alterations is up to them, but generally you can go any way you desire.

Ok then... that told me.

Does that go as far as being able to change your mind about your flight plan while in the air?

You can request a change with whatever air-traffic-controller you're currently in contact with, but again, they must approve it.  Their approval/denial for your request usually is based on weather or other traffic in the area.  If there's not much else going on, they'll let you do whatever you request, but if they have a lot going on, they'll usually tell you to stick to the original plan.