Just above the Dorfman Theatre lobby at the National Theatre, is the entrance to the high-level walkway, a public walkway that overlooks the National Theatre props and scenic workshop.
It's free and is a unique chance to get a glimpse of what happens backstage. I love popping in when I'm on the south bank, as you get a completely different sight each time, depending on which production the workshops are working on at the time.
A beautiful and spacious (outside of rush hour) way to get from Highbury, through east London to south of the river in minutes. The train snakes epically through Hoxton and Shoreditch and opens up the Jubilee Line via the beautiful Canada Water / Bermondsey stations.