Let me see, a number of options present themselves.
World peace of course.
The ability to change certain little moments in time
December 8 1980: "You know John, you really should take the back way in tonight. Trust Me."
Nine months before April 20 1889: Sterilize Mrs Alois Hitler.
Ditto Mama Bin Laden.
1968:"How about you just stay inside away from the window tonight Dr King?"
1968, Ambassador(?) Hotel: "Hows about we clear the kitchens of all suspicious people before the Senator walks through them?"
Mute all the would be Spice Girls in the mid 80s, just in case. Ditto most boybands
1948, The Oval, London: Get Eric Hollies to bowl Bradman a slow long hop to hit to midwicket boundary.
1989, MCG, VFL Grand Final: Get the ball out of the centre down to Gary Ablett for his 10th goal to level the scores, then another quick clearence, resulting in a mark 75m out on half forward by #5. Then a Blightesque torpedo goal to win the flag.
1991, Waverly, AFL 2nd Semi Final: Attempts by Hawthorn to clear ball off half back fail, rebounds to Brownless, who goals, puts Geelong in the GF ,where they win by 12 goals
1992, MCG, AFL Grand Final: Make Neville Bruns hold that bloody mark, plus another goal before half time to go in 7 goals up, and win by 20!
1994-95: Basically the same. Win the GF.
1995/6 Adelaide Oval: David Boon takes his 100th Test Catch, c and b Boon.
1997, FP, Second Semi: The umpires pay bloody Colberts mark!!!
Graeme Hick to get a go at Test level...
Dean Jones not dropped in 1992/93...
And change a few things one has done over time, and a few interpersonal matters like making certain people feel different ways.
Sure I could think of more, but not now.