Given the chance of me being sent to the past, I will probably change one thing and that is to start saving money early in my career. Sure, I could still do it today but if I have started earlier I might be able to purchase a house and lot for my future family already.
Though it may not require a whole mutant power to execute, saving money earlier could have changed my present and eventually my future significantly.
So mutant or not, I would say I could've changed that but given that I can not, might as well start NOW. :)
Also, don't forget to watch X-MEN: Days of Future Past in theaters May 21.