Thames

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.