Having been to London several times, we did a river cruise from Greenwich pier down to the Eye and back. It's a great way to see the sights in London at a very reasonable price. You can get off and visit individual sights, most of which are only a short walk from the piers, then climb back on the next boat. Don't misses for visiting are the Tower of London, Westminster Abbey and a country pub (not one in London). Oh, and don't forget a ride on the Tube if you've never done so.
Read krandco's full Azamara Journey cruise review
Thames River Cruises