play the stock market

Microsoft stock!!!! When it comes out

Well it would be 1962, so I'd probably warn JFK

I would invest,invest,invest in the companies that are big and rollin' today.

Save up a few thousand dollars, buy majority stock in Microsoft and Apple early on, then let them continue operating as before. Stay out of the way of history as much as humanly possible, because only morons change history without knowing the potential repercussions.

Oh and I'd eventually buy out Google too.

First off I'd be bloodly lost without my Android Cell phone but I'd be buying stocks! Knowing what will and won't be successful would make me a very rich man.

As far as helping others, there's not much I could do without potentially changing history, except encourage Steve Jobs to go back to those miscreants in Apple after they booted him out of his own company for example.

What knowledge could I bring without preparation?

That you can use electron tunneling to image surfaces at atomic scales.
The structure of a bucky ball
Carbon nanotubes

That's 3 Nobel prizes right there.

I could also speed up the process of making LEDs, Li batteries and Fuel Cells with knowledge of the material compositions of those devices.

I'd invent Super Mario and buy Nintendo.
Then I'd buy Apple and Microsoft and Sony.

My Grandfather died when I was only 18months old, I wish he could've lived longer so I could've gotten to know him. Meeting him would be top of my list.

Your's was for sure the most touching. Then you could write a book on it afterwards and become a millionaire!