If I had a girlfriend, I'd probably want stuff like "Good morning, (insert cute nickname here). I hope your day is as great as you are" (narcissistic?). I wouldn't want her to worry about waking me up, though. She could text me in the middle of the night just to say stuff like "I love you", "you're incredible (if she really thinks I am)", "I'm so glad you're mine :)", etc.
I'd probably do likewise, maybe also a Bible verse that makes me think of her, lyrics from a song we like, or just a smiley.