You know the Old Gods, how they were supposedly really bad, ruining people's lives for no real reason whatsoever, having their petty power struggles and whatnot.
I would make them look tame.
I think, for starters, I'll appear in several different guises, to different groups, and give them conflicting instructions. For example, I could appear a burning bush to a bunch of people, and tell them not to eat pork or work on sundays, then a few years later, I could appear as some bloke to another bunch of people, and tell them it's ok to do both of those things. Then I'd appear as a voice in the head of some other folks a few years later, and tell them it's not ok to eat pork, and they should declare holy war on anyone who does it, but it's perfectly ok to work on Sundays.
You know, just to see what happens. It'll probably be really terrible for the people on the ground, but honestly, if I were God, I'd probably find that sort of thing hilarious!