What's your favorite hobby to do alone?
If traveling counts as hobby then i would say traveling.
I love doing it alone, it means that i get to do what ever i want, however, and for as long as i want without having to think about if others want the same thing, because its just me.
It also allows me to focus better on my thoughts and myself, it allows me to calm down.
Maybe even being alone could be considered a hobby in my case.