Answering the question "what's cooking" depends entirely on whether you want to take it literally or metaphorically. If you are actually in the kitchen preparing a meal, you can describe the dish you are making. If someone is using the phrase as a casual greeting about your life plans, you can respond with updates about your current work projects, weekend adventures, or upcoming goals. For a witty or playful approach, you can reply with humorous remarks like "just cooking up some trouble" to keep the banter light.