I assume that anyone who visits this site has a list of what they would do if they could travel in time (if not, check Space Moose for some NSFW suggestions). I just added something to my list and I thought I would share it and maybe you'll add it to yours.
Heraclitus of Ephesus, who was known as "the Weeping Philosopher" claimed that you can not step into the same river twice. He argued that by the time you stepped into it again the river into which you originally stepped had flown away and now it was a new river.
Well when I get a time machine, I'm going to go back and meet him at a river and step into it. Then I'm going to travel back in time again to the same time and join him and the earlier version of myself yet again, and then step into the river at the same time as I had before. That means I'd have stepped into the exact same river twice.
Then he'd probably start to weep.