Knitting or crochet. All you need is a bit of string. But seriously one you get the basics there are so many areas to explore. You can of course make things for yourself or others. You can choose easy patterns to relax or more complex patterns to challenge yourself. Some people get into making and dying the yarn while others getbinto creating patterns. Also there is typically a community of introverts that have no problem sitting together to quietly work on their projects.