Greenwich

You could, and I have, easily spend a day there. It depends on your interests and is a nice break from the city, especially if the weather is good. Some of the best views of London from the top of the Park. A list of the major sites in and around the Park.

http://www.visitgreenwich.org.uk/visit-greenwich

Thames Barrier "sail by" trips from Greenwich

http://www.thamesriverservices.co.uk/prices.cfm

Weekend disembarkation trips

http://www.thamesriverservices.co.uk/thamesbarriervisitorcentre.cfm

If you want to visit the outdoor market (Saturdays and Sundays) in addition to things like the National Maritime Museum and the Cutty Sark clipper ship, then you should plan on a full day. Eat lunch in one of Greenwich's historic pubs.

If you are going from central London the best routes depend on what you want to do first:

Docklands Light Railway from Bank or Tower Gateway to Cutty Sark station - closest to the river bank, in Travelcard/Oyster Zone 2

Suburban rail from Charing Cross, Cannon Street or London Bridge to Maze Hill - closest to the National Maritime Museum and Royal Observatory, but may be more expensive as it's in Zone 3.

The railway/DLR station called Greenwich is not close to the tourist destinations.

The Thames Barrier visitor centre is best reached overland by bus from Greenwich.

Remember --the 188 bus-- leaves from Russell Sq and in 15 minutes you are in Greenwhich (Time is on quite Sunday ) very convenient and forgotten by most.

Thanks to you all, this was the information I needed.

I've taken a river cruise to and from Greenwich. It's a pleasant way to get there. Of course, you can always take another way back to London.