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.
I love how much you are aware of the changing seasons out of the city; there are numerous beautiful forests around Hertfordshire to go for a run, a walk or have a picnic. One of the best destinations for a day out from London might be Northaw Great Wood. The main entrance is located on the Ridgeway, the B157 from Cuffley to Brookman's Park.
Audiophile paradise. Boasting one of the coolest looking and beautifully crafted sound-systems I've ever seen. More listening bar than club, it has a low key vibe with a diverse roaster of talented DJ's spinning an eclectic mix of tunes and full albums.