Tu fais quoi demain vs. Tu feras quoi demain

Dan, 

Often, the exercises are written to ‘ work ‘ particular aspects of grammar. The notes on the right sidebar indicate that ‘ future simple ‘ is the main grammatical tense to be addressed in this transcription. 

The repeated use of ‘ will ‘ in English ( not especially casual or informal English either ) is usually done, at least on this site, to indicate the need for the future simple tense in the French translation. On the other hand, for the present tense the form is usually ‘ what are you doing … ’,  or for the future proche ‘ what are you going to do … ‘. 

It can be frustrating at times when the pedagogic goal is specific. Maybe a hint to stick to future simple for ‘ will do …. ‘  on the first screen of the exercise itself would help? 

Although the future simple is reportedly used less in everyday speech nowadays, I have noticed that it is not uncommon, particularly when talking about plans, such as in this discussion.

I agree. There are various ways to express the same question in French (much as there would be in English), even "que fais-tu demain ?". My overall impression is that actual everyday French often tends towards greater grammatical simplicity than learners of the language as a second language might be taught at school. French grammar lends itself to being contorted into various forms, some of which even native speakers sometimes find overly complex, formal, or snobby (or even hard to pronounce). "La langue de la rue" tends towards simpler structures, yet often "slurred" or uttered at speed, which in turn can add to difficulty of comprehension, e.g. "qu'est-ce tu fais" (dropped 2nd "que") might sound like "kestoo-fay", just as "what are you doing" might sound like "wocha-doon" ;-)