I fill my time with the following:
1) Rotating projects; what needs some love that I haven't done for a while? My Github is full of repos I should be maintaining, and once in a while I do so.
2) Run / exercise: this makes me feel whole and keeps my body and mind centered so that I can continue to burn through challenges in my working life
3) Read: I love a good book, even if it's more like a warm blanket (e.g., Dresden Files) than a serious novel (e.g., Invisible Man)
4) Travel; sometimes you just gotta take a break and see the sights This includes one of my favorite hobbies, photography, eating, chilling..