To answer your question, anything that doesn't fall into the typical is fair game. Messages like "Hey, what's up?", "How was your day" and so on are generic. Any message you send should be engaging and entertaining. You should also never send her any marathon texts, and you should also randomize the amount of time between when you receive a message and when you send one.
Some examples of some really spontaneous text messages are:
* "What are three words that describe you most?"
* "What do you want to get out of life?"
* "How many carrots would you give me in order to get a waffle?"
Yes, that last one is mega absurd, but it still has potential. If you are aiming to make the discussion so ridonkulous that she laughs over it, this would be a good thing to throw in. It's all a matter of using your imagination.
The most important thing for you to do is you. You should never do anything that comes off as unnatural or not even you in general. Women will sense this, and they will be turned off by it.
"There is no better than adversity. Every defeat, every heartbreak, every loss, contains its own seed, its own lesson on how to improve your performance the next time." - Malcolm X