I want to get my sister some theatre tickets for Christmas.
It needs to be highbrow, west end/national theatre will be too mainstream. 
No musical theatre.
I used to go to a little 50 seater in elephant and castle a lot when I lived in London but that was a few years ago now.
Don’t know where to start! South of the river or at last not miles into north London preferably.
The more arty/left-field/experimental the better