If you want to visit Greenwich, the best way (my personnal experience) to go from London center is by boat (mbna thames clippers), it takes you right to the Greenwich center where the Cutty Sark and the Royal Observatory are, at the same time, you can explore London by river. Of course, underground can take you there aswell, but the Tube only stops in North Greenwich, fairly long walk to the town center. There are some other way of public transports too, in any case, the Oyster card is a must have, it saves you times and pounds.
Canal Saint-Martin (3)
Formerly carrying only commercial traffic, the Canal Saint-Martin presents lovers of the capital with an urban landscape unlike any other. With its nine locks, its two swing bridges, its Eiffel-style footbridges, its mysterious vaulted tunnels and its ancient trees, the canal offers an opportunity for strollers to rediscover the Paris of an earlier age.
A cruise on the Seine River and Canal Saint-Martin will take you directly to Orsay Museum from Bassin de La Villette taking in the Tuileries garden, Louvre, Ile de la Cité and Notre-Dame, then to Canal Saint Martin with a view, among others , of the Hôtel du Nord. (tourisme93)