There is some wonderful free entertainment available in the Great Hall of the Ahwahnee many of the nights between the 12th and the 25th. The cast of the historic Bracebridge dinners, which take place in the dining room of the Ahwahnee through Christmas, also perform in free concerts in the Great Room on nights where they are not performing in the dinner show.
This year you can catch a concert on Sat. the 19th, a Musical Theater concert on Monday the 21st, and a Christmas concert on Weds. the 23rd.
The Bracebridge Dinner itself is a remarkable event: a three hour dinner and concert that recreates a great medieval banquet. But they are very expensive tickets, and formal affairs to boot. However, if you are lucky with your timing, you may be able to catch a dress rehearsal during regular meal times in the Ahwahnee, and get a much less expensive version of the event. I personally don't know exactly when that will happen, but it will be before the first Bracebridge dinner on the 13th. The concierge at the hotel should be able to let you know more details.