i would create a universal church that floats somewhere in the milky way. i would have my friends, family and ppl i like able to come and go and visit me and allow them to come and go through teleportation any where in the universe.
i would allow mankind a second chance to improve themselves giving everyone enhanced awareness and entelligence low level telepathy.
i would reset the eco system to where the planet would have more then sufficient food both in plants and animals that would keep ppl from suffering and starving at least for a few centuries allowing ppl to choose whether they harm the planet by depleting its food source and other materials or find alternate means.
i would answer prayers every decade or so, so long that they arent selfish in nature, maybe even answer one large scale prayer that would effect the world so long as the request was properly thought out taking all consequences into account.that would be my one miracle to the world and would be on behalf of all ppl and believes.
although i would not reveal my existence to the world by a physical body to tell them i am real, i would cause localize earthquakes to all parts of the world that would kill all extremist and their churches at once. allowing the world to decide the significance of that action.
other then that i would create a heaven like in the movie where dreams may come where each person is responsible for their own heaven or hell. and i would oversee it and randomly interact with ppl who would choose to speak to me, so as not to destroy their reality.
other ppl can enter one anothers reality and can leave it to join the central one where all soul share.