Betty Blue eyes is supposed to be great fun, it is the musical version of, I think, an Alan Bennett story. The Woman in Black is a scary ghost story by Susan Hill that has been going on for 20 years, but is truly excellent has a spectacularly frightening moment that is a wonderful piece of theatre. Last minute tickets for that should not be hard to come by. One Man Two Govnors at The National, is the thing to see, but fully booked, however they might have returns on the day. And there is a Pinter play at the Comedy theatre, Betrayal with Kirsten Scott Thomas in it, I don't know how easy that would be to get into. If you want something hilarious with belly laughs, and you have a sense of the ridiculous, my favourite is The Thirty Nine Steps at the Criterion. It has three people playing the entire cast of the John Buchan novel, and it is fabulous. Enjoy your visit.