In no particular order, piano, any number of languages, higher math, writing novel-length fiction, web development, fine carpentry, Aikido, Tai Chi (as a martial art,) yoga, sculling, psychology, walking on my hands, dancing in all styles from ballroom to tap to break, sleight-of-hand magic -- wait, are there any skills I don't want to master?

Why? Why not -- everything I learn expands my mind and, often, my body, extends the range of my experience, and, hopefully makes me wiser, kinder, and more powerful. Who wouldn't want to master every skill he or she could?