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.
This is a great, free and lesser known spot in London. A glass lift takes you up the side of st.Paul's cathedral right out onto the roof terrace of this shopping centre. A great view over London.
In my opinion this is the best bookshop in London, it stocks all my favourite magazines and has a wide selection of photography books, I spend a lot of my time there just browsing through all the books they have. David Bailey once described the place as “maybe the best photography bookshop in the world”