Curate, connect, and discover
Funnily enough I think Geto has much more chances of alter the course of time than Gojo does, because when you think about it the story is written in way that puts Suguru as the main character who lives most of situations while Gojo is absent or doesn't have enough context to actually prevent stuff to happen.
So Geto could change a few inmediate things that happen when he was a teen such as:
_The death of Haibara (that's his name?)
_The death of Riko.
_The whole fight with Toji to ever happen, thought it probably will cause some troubles as Gojo will take more time to get to the 100% of his power.
_Probably can do something about the two sisters before their situation gets out of control. I guess he could pretend he's going to take them to a "school for gifted kids" before they manifest curse energy.
_Guess he indirectly will cause Megumi to never meet Gojo and became a sorcerer? Assuming Toji doesn't die here and Geto probably doesn't want to have anything to do with him or Megumi, is unlikely he will do something for him.
And on the more long term side when he was an adult he could do more stuff as well:
_Probably go after Yuta and while he's mad because of his original death, he probably could prevent Rika to go crazy that one time.
_Assuming Suguru is aware of all things Kenjaku did and his plans, he probably could be actively hunting him knowing several things. Still will be difficult deal with Ken because he's still sneaky.
_Geto probably could save Junpei from the plans of Mahito and instead adopt him into sorcery (this is my fav headcanon btw).
_Doing something about Yuuji, maybe find him before eat the first finger and explain him the truth about his parents.
_Shibuya will be totally different of course.
Suguru wakes up in his teenage body, during his first year of Jujutsu High, with his last memory being of Satoru getting sealed inside the cube.
Like hell will he allow that to happen twice.