You could also get a boat at Westminster (by the London Eye, City Cruises, or on the other side of the bridge). Get a return to Greenwich. It takes about an hour, sit outside for the best view if its not too cold (wrap up warm, its worth it). You will get a running commentary by the boat's skipper, and they are usually very funny. Very interesting, I do this every few months. You can stop at the Tower of London, either on the way there or on the way back. Greenwich Market has a good Craft Market on certain days of the week, google it to find out the best day to go. Also very maritime if that interests you. You can get a drink on these boats, not a 'meal' though. Best to go early, you will be able to do more. Enjoy !