Well, it'd really just be the same thing I talk to Him about, so anything and everything. Pages long worth of emails, or sometimes just a couple of words/sentences.
I'd probably send Him a lot of questions, since I know He doesn't get tired of them (probably the only one who doesn't, ha) and lots of pictures of His creation, since I know He'd appreciate them as much as I do.
He'd probably send me back gentle, sweet responses with words so beautiful that I'd have to read it 5 times over and still not be able to soak it in. Then there are times when He would send a firm rebuke or correction, and I'd make myself read it even if I didn't want to.
He'd also have an uncanny knack for sending me just the right words at just the right time.
And He'd never, EVER send me those silly "If you don't send this to 42,003 people in .60 seconds then Maggie the Murderer will come in the night to kill you!!1!!" emails.