The best way from Central London to Greenwich is by Thames Clipper. If you feel energetic, get off at Canary Wharf, take the ferry to the Doubletree Docklands Hotel, look at the two docks there, then follow the waymarked path along the river to Greenwich, under three miles. If you do take the DLR, get off at Island Gardens for the view as painted by Canaletto, then walk through the tunnel to the Cutty Sark.