For anyone who is a fan of the National Theatre on the South Bank I highly recommend booking on to one of the theatre’s backstage tours. When I went a few months ago with two friends we lucked out by being the only three people that had booked that particular time slot. However, I imagine it to still be a great experience, even with a full tour of 20 participants.
Go on the tour to hear some great facts, such as why the building is so plain compared to the traditional West End theatres, see some tricks of the props department and see the sets of one or two of the current productions. There are a few other tours as well as the Backstage tour which I imagine are equally good.