What are some great hobbies for an introvert?

Photography. I took this up as a hobby after having kids and no longer had time for woodworking. It is portable and doesn't require expensive equipment, although if you get into it you can certainly spend as much as you like on lenses.

I can bring the camera with me on vacation in a city and take fun pictures of buildings, or get lost in the woods for a few hours observing and photographing nature.

My photos are just for me, but there are often small local photo clubs where you can share, learn and even exhibit in coffee shops and libraries if you want.