I really enjoyed watching the Royal Institution Christmas Lectures, presented by Chris Bishop. I don't think I've actually watched them properly since I was a kid!
We sat down as a family every night to watch and it clearly sparked some interest from the kids. We've had the cover off the computer looking at its innards, we've been playing with Phun (the kids remembered it after seeing the multi-touch displays), we played with GNOME's Dasher. Heck, we even touched on public key encryption.
Sure, there were a couple of bits that weren't so good. In particualr Bill Gates was seriously dull and the programme on software, The Ghost in the Machine, was not exactly great (the kids still don't know what I do ;-)) but over all, fantastic.
If only all TV was that interesting!