The Greenwich Observatory with its green laser beam can transfigure any night sky. If you decide to cross the Thames via the foot tunnel, make sure you look out for the green ray. You can be on one side of the meridian in a minute, and cross to the other side on the next, without even realising it.
Travelling on these uncrowded river buses is such a relaxing way to see the city's landmarks from the Thames. A really quick and lesser-known way to travel from central London to Greenwich, especially on a sunny weekend.