Formal:
* 你在想什么？ (nǐ zài xiǎng shénme?) - This is the most direct and common way to ask "what are you thinking?".
* 你心中在想什么？ (nǐ xīn zhōng zài xiǎng shénme?) - This is similar to the previous phrase but emphasizes the idea of what's in your "heart" or "mind".
Informal:
* 你脑子里在想什么？ (nǐ nǎo zi li zài xiǎng shénme?) - This literally translates to "what are you thinking in your brain?", but it's a common and casual way to ask.
* 你脑袋瓜里在想什么？ (nǐ nǎo dài guā li zài xiǎng shénme?) - This is another informal and playful way to ask, using a more endearing term for "head".
Other options:
* 你心里有什么想法？ (nǐ xīn li yǒu shénme xiǎng fǎ?) - This asks "what thoughts are in your heart".
* 你在想些什么？ (nǐ zài xiǎng xiē shénme?) - This is a slightly more formal way to ask "what are you thinking about".
The best choice depends on the situation and your relationship with the person you are talking to.