It really depends on what sort of girl your dealing with and if you've already F closed her. I came up with this on the spot 2 days ago after I kept getting texts every 5 hours from a girl who I met on the weekend asking what Im doing (havnt F closed her yet). and the convo went like this.
Her: Hey what are you doing
Me: Nothing really, just got home, what are you doing
Her: Im just at my mums bored has hell!
Me: Lets play a game then
Her: OK

what do I have to do
Me: The rules are simple, we send 4 words at a time to each other to create a short story... you start
Her: OK, there once was a
Me: there once was a man who liked eating
And so on and so forth. we came up with some pretty hilarious stuff, with plenty of sexual remarks in there, and most of them where from her

Give it a try