I attended this yesterday. Knuth started with a problem about rectangles inside rectangles (
https://imgur.com/faWRt2q — it's going to be an exercise in 7.2.2.1 of TAOCP when that's published, currently in the draft version of Pre-Fascicle 5C). He worked through some small cases, made a conjecture, showed a problem submitted to the Monthly, and lots of cool stuff with generating functions. The lecture was also peppered with jokes and cool stories, including a fascinating conjecture by Bill Gosper, who has a long history of coming up with these Ramanujan-like identities. He also showed a wonderful conjecture (involving queens on an infinite chessboard) that he thinks may never be proved, and showed a snippet of his CWEB program for the problem.
Knuth is turning 80 in about a month, and his talks seem to get better every year. (Though last year's lecture on Hamiltonian Paths in Antiquity, which among other things covers Sanskrit poems that satisfy a “knight's tour” constraint, holds a special place in my heart — been planning to elaborate on it.)