Thames River cruises

1) one of the HOHO busses has an optional boat trip from Tower to Westminster (London eye)

2) your oister card can be used for trips on the regular "boat bus". I recommend going to see the Thames Barrier.

3) don't forget the canal trips from Little Venice to Camden Locks and London Zoo

Don't know how much you want to see along the Thames.
We took the boat from London to Greenwich and it was a really lovely ride, and a chance to see parts of London that I had not been to. (boarded the boat at the dock next to Westminster Abbey.)
It was just long enough - we then toured Greenwich on our own and went to the
observatory and of course had to stand atop the Prime Meridian line.(of course we took our picture there)

You can pay for river bus fares using Oyster pay-as-you-go money, but they aren't covered by Travelcards.

There's no tube service to Greenwich. The Jubilee Line goes to North Greenwich, which is nowhere near Greenwich proper and is estate-agent speak for the area around the O2. Quicker ways back from Greenwich after the cruise are the DLR from Cutty Sark, or rail from Maze Hill or Greenwich stations.

Phillip, Thanks for the correction -- am deleting post with the mistake.