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.
My all-time favourite place. Whitewashed ex-smokehouse where Fergus Henderson pioneered the resurgent interest in offal dishes. The restaurant is brilliantly unfussy, retaining lots of the smokehouse’s original features, and the food very British, making St. JOHN something of a London institution. The restaurant has a winery and bakery.
Old townhouse style building set in front of juxtaposing high risers makes for an odd placement, but internally it's a quirky spectacle. One half is dedicated to the sale of antiques - anything from old door handles to hallways lined with hunting trophies. The other half is a relaxed bar and hidden gourmet dining area constructed of similar furnishings [all for sale too]. Even if you don't wish to spend £'s, a walk around the place is advised.
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.