When forming questions, the word order is changed, an auxiliary verb is often introduced, and a question pronoun such as "what" or "who" is used.
"I do things on Sundays." is changed into "What do you do on Sundays?" The auxilliary "do" is introduced and "things" is changed to "what."
"I am doing something on Sunday." is changed into "What are you doing on Sunday?" The be verb "am" is changed into "are" and "something" is changed to "what."
The first sentence / question ask about regular or habitual activities. The second is asking for a future plan or activity. The present continuous is often used to talk about the future, and often an adverbial is added for clarity: "I am flying to New York on Sunday." or "I am cleaning my home on New Year's day."
As a side note, the use of present progressive to talk about the future started being used around Shakespeare's time.