Greenwich is a cool town, on the east side of London on the River Thames.
It is probably best known for being the home of the Greenwich Meridian - marking 0 deg Longitude. This is the same place from Greenwich Mean Time fame!
The meridian is marked by a metal line on the ground at the Royal Observatory, so you can stand in th East and West Hemispheres at the same time.
An easy and pleasant way to get to Greenwich is to take a river boat there from central London - I think that they leave from Tower Bridge and The Houses of Westminster.
Greenwich also has the Cutty Sark - the world's sole suriving tea clipper - a ship that was originally used to bring to Britain.
Just one tip, based on experience, during the Winter, the river boats last departure in the day is pretty early - before 5pm...that caught out a piggy on vacation!