Sales. Learn to sell something. If you're taking time off, why not consider doing phone cold calling for a charity. This is particularly brutal but probably a sharp, free course in improving your sales and cold calling skills.
Everyone needs sales skills, whether it is to understand why people buy, or just to 'sell' ideas, whether pitching to investors or convincing a partner/superior or team to implement your idea.
Everyone needs sales skills, whether it is to understand why people buy, or just to 'sell' ideas, whether pitching to investors or convincing a partner/superior or team to implement your idea.