Keep text conversation going?

Were 14. Me and this girl really like eachother and with it being summer I don't get to see her that often. I've got a phone and she's texting from an iPod so she doesn't always reply cause she doesn't have wifi, but anyways both of us aren't really having anything going on during our summer so we can't talk about that. I know huge secrets of hers, so that's not an option. And asking her how her day been is what I do when I start talking. We can keep conversations going, but we start to run out of things to talk about. What can I do or say to get a conversation going when we run out of things to say??