Keeping Sharp

A side-effect of having graduated, I think, is that I seem to spend less time forcibly stimulating my mind. I mean not just absorbing, but really working through new material and understanding it. I have certainly learned a lot in the past few months, but I don't think very much of it has the same quality of information stored as when, say, I spent a week diagramming and memorizing how respiration works in excruciating detail for a physiology class.

I know that basically everyone experiences this, and none so much as those involved in academia, but I am really a bit unsure what to do about it and I'm already a little worried about what veering away from "hard science" for a little bit to do my Masters might do to me. I just want the quality of my brain to remain unhindered as I move through my adult prime. I've been watching a lot of TED talks, reading more, and trolling through Khan Academy videos on random subjects just to keep fresh, but somehow even that doesn't seem quite enough. Don't get me wrong, I could be trying harder, seeing as I also sometimes decide to watch 12 episodes of Community back-to-back and call it a day, but still. How do you guys keep your minds keen?