Great Scott!

I saw this prank done in a movie but I think it could work pretty well in real life.

Look up an eccentric inventor in the phone book. Then go to his house in the middle of the night and claim to have arrived there from the future in a time machine that he invented. When he doesn't believe you, display knowledge of an invention of his that will be used in the time machine (this prank takes some research but you could probably search using a website like www.google.com or something) (also, it's best if this inventor is actually attempting to invent a time machine). He might not realize that the information is commonly available and will therefore believe you and think that you really are a time traveller.
fin

This prank might not be funny, but it will make the inventor feel good about himself because he will think that his life's work was a success.