I'm not sure there's another activity I find as immersive as creating music.
I enjoy other solitary activities like reading and walking. However, I'm a slow reader and feel burnt out after a page or two. With walking, I often appreciate the feeling of motion and the transitory appearances, but going down the same old inevitable paths can feel very boring.
However, with making music, I feel as far away as possible from worries and distractions. There's a lightness to it that I don't find in other activities.