Writing Prompt: Time Travel

Thanks to Shiv for the following idea:

Writing Prompt: You can go back in time for one day, and while you are there, you can tell anyone, including yourself, anything, possibly changing the future. When do you go, who do you talk to, and what do you tell them?

And if any of you have any ideas for interesting prompts, shoot me an email and I'll post them and give you credit.