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.
Great selection of books and magazine with a big focus on rare. out-of-print books Japanese photography. Can sends hours in this little shop. I'd try to avoid on Market day (Saturday) if you want to have a good look as it can get a bit busy for such a small space.