I won a Silver Travel Advisor prize of a ride for 2 on the Thames Jet in London. I took my prize this week and it was an ideal way to cool off in the heat of London. The journey passed the interesting buildings along the Thames to beyond Tower Bridge and then they opened up the throttle and did exciting manouvres with the boat. It was really good fun if you fancy treating yourself to the 3/4 hour trip.
If a standard boat trip along the Thames is more your thing Thames cruises offers good hop on/hop off type trips from Westminster Bridge to Greenwich and back, which is good value.