I'm guessing you only put "the" when it's that specific breakfast which would make the sentence "what are you doing before THE breakfast" as in that particular one. If you miss it out "the" it could be any breakfast, so the sentence would be "what do you do before breakfast" in general
I'm not sure, but that phrase would probably sound too ambiguous. "Prima colazione" means breakfast. "Colazione" also means breakfast by itself. Your sentence - if I'm right - would literally translate to "what do you make breakfast", which is probably not valid Italian. Could be an idiom, though. Please correct me if any of this is wrong. Thanks.