It’s a fairly straightforward question. Direct revelation (talking burning bush, miracles such as parting the red sea or raising the dead) is out. That leaves no reasonable room for doubt.
Let’s say you wanted to be more subtle, yet clearly suggest your existence to humanity, how would you do it?
I don’t understand why direct revelation is out. If I wanted people to believe in me, I’d appear to them and prove my power. If I didn’t really care one way or the other, but I wanted to make a little game of it, then I’d go with the subtle stuff.
I’d create a universe that once set in motion , without any further interference on my part, and using only the self consistent set of natural properties with which I endowed it, would end up resulting in creatures that could ask questions such as the OP.
I’d mysteriously eat socks out of your dryer, maybe for kicks and giggles show up in the middle of church say it really was Adam and Steve.
I’d be lobbing thunderbolts at those that do stupid things in My name.
Fred Phelps? KABOOM!
Pat Robertson? KABOOM!
Well, that is what I’d do if I had a choice; I have no admiration for faith.
If I had to be subtle, I’d do what was done in Carl Sagan’s Contact; write messages into the universe’s physical laws.
I thought the answer was obvious: It’s out because this topic would have no reason to exist otherwise.
Or, if you’d like a less frivolous answer, let’s say it’s to give people the ability not to believe in a superior power.
So, what subtle stuff were you thinking of? That’s what I’d like to know.
I’d make man inquisitive and intelligent and direct him toward the sciences. Once he digests the most basic tenet of causality, he will know that something had to set everything in motion. I would also tease the non-believers by giving them a very vague inkling into the inner workings of matter and the universe and allow them to muse that such knowledge points to an existence that can subvert the basic tenet of causality. After all, I like to have something to amuse me. Eternity can get boring if you let it.
Israel seemed to manage during the Exodus, so I don’t buy it.
But if you want to be subtle, how about natural disasters mysteriously changing course and missing inhabited areas at the last minute, consistently. It could be chance, but not likely, and it has the advantage of God stopping killing people.
Symmetry.
Or Bach.
Well, if I have to… 
I’d probably go with personal suggestions of my presence. Something completely subjective. I’d basically turn myself into a metaphor that individuals could associate with certain places or ideas or people and experience me as they please. If I’m not going to make it clear that I exist, I might as well let people define me in any way that suits their needs.
Daily keggers!
True love.
Would I be “a” godette in a bunch of gods and godettes, or would I be God? Different concept altogether.
Id appear before the amazing randi whenever nobody else was around…just to screw with him.
I’d inspire a holy book that doesn’t mention dinosaurs, then hide a bunch of dinosaur bones where people can find them. And I’d make the dinosaur bones older than the age the earth is supposed to be in the book. That’d really throw 'em off. Oh, and I’d put DNA inside apes that’s 96% identical to human DNA, then claim that humans and apes have no shared ancestry. Let ‘em figure that one out. Subtle enough for ya’?
Thing is, he’s trying to expose fakes; as far as I know he’d have no problem with the real thing. He might even ask you to hang around and help him embarrass fake miracle workings by showing them the real thing.
I would imprint an image of myself in a bagel.
I’d appear as a giant face in the sky and personally appologize for all the suffering I had caused.
i’ll take away the ladders from swimming pools when no one is looking.