Investing in skills as a developer is a great way to benefit both yourself and your company. You have a great gift -- that of "free time at work". I had lots of this in a former life as a sysadmin, and I squandered 90% of it, and I deeply regret all of those micro-decisions to surf the web instead of learning a skill.
Find problems which interest you (for me, that's currently lexers, parsers, lisp, code visualization, tools to make tools to make tools, etc), or parts of the "stack" which you never fully understood (In college we never actually went over how a C function call works, how to read stack frames, etc).
If you don't have any problems at hand to fuel your curiosity, maybe try upping the signal-to-noise ratio of your junk food -- read a programming book instead of reading HN. Small, consistent investments are more important over the long run (30 min / day for months rather than a week-long fugue state).
If books aren't your thing, find a structured series of exercises which you can work through. Project Euler is great. Make-a-lisp is great as well (github.com/kanaka/mal).
Turn what you learn into blog posts, github gists, or flash cards -- distill your knowledge into easily digestible parts so that you can catch yourself up to speed quickly 6 months from now when you need that topic again.
These won't create immediate benefit, but five years from now you will be tremendously more valuable.