I have always been asked this question but I want to be sure of the correct form. And does the correct form of this question mean "what are your plans for today?" Or "what will you do today?".
Thank you for any help
Dictionary of American idioms
be up to something is the correct sentence.It means: To feel strong enough or knowledgeable enough to accomplish a certain task. Example, Are you up to preparing dinner for five hundred people? . A second meaning is: Tendency to do something mischievous.Example, I'm afraid George is up to one of his old tricks again. A third meaning is: doing something. Example: What are you up to right now? So, to answer your second question, it can mean, "what are your plans/what are you doing today", although it sounds more natural to use it when asking someone what he is doing right now.
Up to is common way to ask what are you doing/busy with?
up to (adj) - Busy or occupied with
What are you up today? does not seem correct to me. The correct usage is up to.