Hyde Park by d_d
Hyde park is a very nice place.Maybe this tip should stay in must see activities section but when I was in London Hyde park was my relaxing corner.
If you want to relax, take the tube and go there and stay in the sun lying on a deckchair (a nice man will ask you 1 £ for the chair), looking the local people skating, jogging, playing guitar, studying...
You can do whatever you want in a peaceful place.