What is on your mind? is an idiomatic expression meaning, "What are you thinking about?" Also, I have a lot on my mind usually means that I have a lot to worry about at the moment.
Originally Posted by peter123
What do you have in mind? is another idiom that means, "What are you thinking about?", except this one has the connotation of "What's your idea?" For example:
A: Are you busy tonight? What is in your mind? is only used in a very literal context - I can't think of a time that we would say this in normal conversation! (Maybe someone else can...)
B: Not really. What do you have in mind?