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.
Although my skating days are now over, Stockwell Skatepark located just behind Brixton Academy is one of the reasons I moved to Brixton. It’s an excellent place, lumpy and hectic but totally friendly with decent Portuguese coffee and cakes just next door.
A brilliant little hidden gem of a place to relax and look at some interesting rare / Out of print books and Magazine. Their evening art events are also very special. Great atmosphere and great people.