I don't know for how long it has ran, but Peter Neumann started the Kinderseminar at Oxford, specifically for graduates to talk about anything they wanted. The recent past events are [here][1]. I used it to talk about algebra mostly, but also combinatorics, cryptography, electrochemistry, social choice theory, and a few other even more random things. Allegedly someone once gave a seminar on the laws of cricket.

It was running since at least the 1980s, probably earlier. I think I remember him once saying that he brought it back from Reinhold Baer's Kinderseminar that ran in Oberwolfach.

After Peter retired, I ran it for a while, until I left Oxford, and it was in turn passed on. It appears to still be running, albeit with a presumably Covid-related suspension. The main original feature was that it did not take place in the departmental building, but in a College office. This tradition at least appears to have now been lost.

  [1]: https://www.maths.ox.ac.uk/events/past/657