Restore the Eastern Roman Empire.
Get myself some self confidence.
Bring back BIONICLE with G1 level story and G2 level sets(+G1 vehicles and stuff)
Make myself a cool robot suit to mess around with.
Magically increase my programming skills.
Make myself at least somewhat likeable.
Make Psynergy real.
Other shallow stuff.
Fix the environment.
And the most important thing: Make everyone on the world hear a loud snap, and give them all enough knowledge about Marvel and the Infinity Gauntlet to think that half the world is gone, but actually nothing else happens. Watch what happens. If it's too bad, reverse time a bit until before I did it.
Also, slight tangent, couldn't Thanos just, y'know, make more resources? Instead of halving the people, double or even triple resources every time people are about to run out.