You can say it 오늘 뭐 할 꺼야? (what are you going to do today ) or 오늘 뭐 할 예정이야? (Is there any plan to do today?) 오늘= today 뭐=someting (to do) 할=be going to 꺼야= asking form. In Korean, subjects, and objects are frequently skipped. because If you use those sentences to someone, he will know subjects and objects with context. thank you.