Are you looking for some in depth study on a particular subject or just something to kill time?
It really comes down to what your interests are... especially when you are looking to kill time.
In my free time (or in between long boring meetings) I usually enjoy learning about universe (stars, planets, etc.) The place is so full of mystery, it's amazing.
Moreover, I use "Space Engine" space simulator software that really helps on this subject. Looking at different stars, how their planetary system adopts to them. Or looking at a black hole and see how stars zoom past it as they get near to it following their orbit. The possibilities are endless!
If you're not into that stuff, maybe you can catch up on latest tech and gizmos. Gizmodo and Engadget are a good source of info and have interesting small articles (their sister sites such as Kotaku, Joystiq, lifehacker etc are good too).
Another part of my new developing hobby is photography. I check up on good lenses and different techniques of photography. Since I have a huge boner on space stuff... space photography is my main focus... and so is macro since everything looks alien with macro photography!