To have in mind: to have an idea brewing in your head
Alicia: Well, what are you going to do to get your boss to give you a raise?
Meghan: Well, I have something in mind.
Alicia: What? You have a plan?
Meghan: Yes; I'm going to threaten to quit! Maybe then he'll give me a raise!
What's on your mind: a question asking about someone's emotional status
Alicia: Hey Meghan; you look glum. What's on your mind?
Meghan: Oh, nothing. It's just that my boss suggested that I quit after I threatened to leave the company.
Alicia: Does that mean you don't have a job anymore?
Meghan: Yes, it does. I worked so hard to become the clerical assistant; now I have nothing.