Practical? Probably August 2004. I'd give anything to redo high school (the academic part anyways).
Potentially future-altering? Not sure...many things people might think to do to reverse some pretty horrible things but then the present that I know it as would likely change...I'd just go to different time periods and see what it's like to live in each one for a day... of course I'll need to hit up the costume shop first..