what will you do this weekend 单纯问将来发生的事,比较正式；如有选项,will可有情态动词的意思（愿意）例如
What will you do this weekend,clean the house or water the garden?
周末你愿意干什么,打扫卫生还是给花园浇水?
what are you doing this weekend 现在进行式表示将来只用于口语,且说话时离将来时间（这里是“周末”）很近（例如星期五说）
what are you going to do this weekend 问对方主观打算；I am going to do something不如I will do something那么肯定.