"What are you doing?" It's simple enough to us, but baffling to students who haven't learned that it refers to both present and future tense! What's more, they'll often over-use "will" for future, which is less conversational. Here's a ppt with different future times, to practice forming the question in order to make plans. It's one of the most common English questions, so emphasize how amazingly useful it can be in order to find a boyfriend / girlfriend!