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.
2 Willow Road designed by Ernő Goldfinger and completed in 1939. It has been managed by the National Trust and is open to the public. Goldfinger lived here with his wife and their children until his death in 1987.
I go to the Heath every weekend (often via the wonderful Waterlow Park which skirts Highgate Cemetery) and during the week too if I’m working from home; it helps to put things in perspective when I’m thinking about my work. It’s great in the summer for obvious reasons but I love it the most in autumn and on those rare days it snows so much London grinds to a halt (which doesn’t take much). Everyone heads up there to make giant snowballs/cats/creatures and surf down Parliament Hill on anything they can get their hands on. Grown ups turn into kids for the day. My husband and I made this snow yeti last year, it was sad to leave him behind but he soon made friends and even made it into the Camden New Journal.