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.
It's such an iconic building and it's always a joy to be anywhere near it. It just dominates the landscape around it. Ever since I first saw the cover of Pink Floyd's Animals I've loved it and I still do. There are always reports in the news for plans for developers to spend billions on refurbishing it. I'd genuinely prefer it to remain as it is rather than see it being turned it into a massive shopping centre full of chain stores. The thought of it being turned into some kind of Westfield full of people on Facebook on their laptops in a Starbucks fills me with rage.
I grew up around this area - it really is a gem of a place. The interior is so cool, with sort of organised clutter everywhere. They have a little garden and they serve food. Its not the best in the world, but the whole experience there is great.
This is one of the only platforms on the London Underground that is not bombarded with billboard advertising. It's a reminder to myself of how an uncluttered environment can give you so much more room to think.