Here's the quintessential brief entry: if you have access to the Science Channel in the US, and have nothing else to do today (Sunday 22 June 2008), there's a marathon running of one of the best science documentaries of all time, Carl Sagan's Cosmos. I've watched the whole series several times and own it on DVD (having replaced my videocassette versions, if that tells you how important this show was and is to me), but I have it on as background at the moment (skipping through some of the most irritating commercials ever). This presentation features some updated graphics from the original 1980 version, and linking narrative by Sagan's widow, Ann Druyan. Here's the link to the inevitable associated website.
You can, of course, watch much of it on You Tube, Google Video, and elsewhere. I've included an example, Episode 1, part 1, below. But if you want to just sit in front of your telly and do something useful for your brain and yourself, you could do far, far worse than this.