At least in terms of decisions effecting the rest of you poor earthbound peons... There are almost certainly plenty of other changes I would make elsewhere in the universe.
1.) Redesign the species here on Earth, humanity in particular. Not necessarily to the point that the final product is unrecognizable, but I could name hundreds of critical design flaws I would prefer to correct. I'd also toy around with the idea of uplifting a few other species to full sentience, but would require significantly more information first.
2.) Create a series of superconductive space elevators to be exploited for both energy production and transportation.
3.) Create a functionally unassailable station that automatically and unerringly prevents anyone from using weapons beyond a specific destructive limit (all types of wmds would qualify, as would most major explosives), its secondary function would be to stave off major environmental disasters (IE super volcanoes and major meteor crashes). Its not there to prevent all ills, just to keep matters from getting out of hand.
4.) Set up an afterlife system not at all based on faith. Those who live good lives (with the primary definition of "good" simply being the ethic of reciprocity) can spend as long as they like in a place of peace and tranquility, with the option of at any time being reborn again in the "real" world with vague nonspecific memories of their former self. Those who don't do so undergo temporary remedial punishment before joining the rest, with the worst punishments being for those who abuse power, especially power of the religious variety.
5.) Most importantly, after this is done, I tell everyone, in person, what the rules are and to not expect any further involvement from me, in terms of either assistance or interference, much less some future apocalyptic scenario. I would prefer to prevent religions springing up around me entirely, but suspect that it would be an uphill battle to do so without warping everyone's mind.
6.) Retire, at least so far as Earth is concerned. I've no interest in being some kind of overlord, and would prefer that humanity finds its own path to greatness rather than shoving them along myself.
