The Regent's Canal has been London's other water highway since 1820. In a walk on the canalside you will see a canal tunnel, wildlife, boats, and the remains of a past age of tranport. It is a peaceful place to walk, leaving the nearby city behind.
At the east end of the canal in King's Cross is Islington Tunnel. London Canal Museum organises occasional boat trips through it, and the only way you'll go through is by boat, because there is no towing path. You can walk over the hill, through Islington, if you want to go east. Through King's Cross the route is of course urban, but surprisingly peaceful. See the old horse ramps in the side of the canal, where once, horses would be led out after they fell in the water. You can walk along to St. Pancras Lock, the only lock in King's Cross, and, if you're lucky, you'll see a boat going through. Beyond that, Eurostar trains pass overhead and you can climb the steps to Camley Street, where a fine natural park occupies the site for a former coal yard, supplied by railway wagons on a gantry over the canal.