Hmm, a good excuse to re-watch Season 1!!!
Right off the bat, one of the biggest things I think he'd want to change is Samuel Platt's murder. He felt guilty about that, and I'm sure it's been hanging on his conscience.
I think Lois might get suspicious of Clark having so many insights into their investigations, and it might end up drawing her attention much closer than he'd like (and not in a good way!)
