The Docklands Light Railway is, as far as I know, completely above ground (I haven't ridden the part past Greenwich yet, so I can't be absolutely sure).

As for a tour of the dock area, there isn't much dock area left. The area where the old docks were that handled the world trade of London are largely gone, replaced by modern high-rise buildings of flats and offices.

Greenwich is an interesting stop, with some old naval and maritime exhibits, and, of course, the old observatory on the Prime Meridian contains a small museum.

You can also walk through the tunnel under the River Thames to the DLR station on the other side.

Regards...

Dave Hatunen Tucson AZ