If you want a peaceful day out of the busy city, Highgate doesn't really feel like London. The cemetery is interesting and peaceful, there's a tour of the old half which is definitely worth the money. Karl Marx, among others are buried there.
Sometimes when I wake up it feels like I am at sea watching a big ocean liner pass by. It is the view from my home in one of the two stepped concrete apartment blocks that make up the Brunswick Centre in Bloomsbury. When you visit skip the chain stores of the shopping centre below (except for the excellent Renoir Cinema) and instead get an invitation to one of the small but gorgeous flats with their winter gardens or just wander through the spectacular concrete A frames and across the vast sun drenched terraces on podium level.
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.