a personal skill which improves life over time, e.g. take up meditation, tai chi, yoga, regular fitness or similar

If planning on going freelance/solo develop presentation skills. (reading up on stuff like https://www.amazon.com/gp/product/0321668790?ie=UTF8&tag=gar...)

For technical skills, really depends on where your interest and skills are ... look for longterm trends and learn stuff that matches your personality/interests/skills