Hyde Park, together with Kensington Gardens to the west, is the largest of the three royal parks. On the northeast side is Speakers Corner, where people are free to speak their mind--as long as they dont debase the Queen. Hyde Park is home to a lake called The Serpentine.
You can hire boats or even take a dip in the water. The lake has two cafe/restaurants, the Lido cafe (Next to the Diana Memorial fountain) serves fantastic homemade style hamburgers. If you fancy a walk, it takes approx 1.5 hours walk around the Serpentine.