In regards to the tube, does anyone know what Greenwich is in? Oyster cards only cover Zones 1 - 6.
Greenwich is on the Docklands Light Railway, which is accessible through your Oyster card. It's on the border between Zone 2 and 3....I think it's considered a Zone 2.
Here is another idea........
Take the ferry/boat from the Westminister Pier to Greenwich...it takes just about an hour and you can get some wonderful photos of the Tower of London, the Tower Bridge and many beautiful sights....
The ferry lets you off right in the heart of Greenwich and it is quite enjoyable.
The hop on and off buses include the boat in the price of the ticket........
We planned to take the cruise from Westminster Pier to Greenwich but the last one leaves at 5:00. We really wanted to come back after dark! Therefore, we thought we would take the metro back unless someone has another suggestion.