Honestly, what's on my mind half the time is fun stuff like Pokemon and video games, because I'm a kid.
Kids should be kids. And most kids' lives are all about fun.
Thinking is a beautiful thing. You can imagine anything you want. Anything in the world. There's no law against it. You are free, in your mind. That is, if you know how to control it. My dad tells me that your mind can be your friend or your enemy. Your servant, or your boss. Your mind can drive you crazy, or it can turn you into a spiritual master. My mind is not very controlled yet. I have a weird fear, and sometimes, when I start to think, Oh, this fear is about to come true, my mind makes me more and more worried, until I start to believe it. But then, if I get distracted, I start to feel fine, and I realize it was just my mind playing tricks on me. I hope your mind is your servant.