The second day of our London trip, we started out with the Thames River Cruise. Perfect since it was rainy and windy and that way we'd get a cross section of the city while being shielded from the weather.
The cruise was surprisingly short. From Tower hill to Westminster in a little under half an hour (not counting the wait for the boat) and sadly the view wasn't that fantastic from the boat we were on.
You can take the boat further than tower hill and it goes in two directions. Compared to the 'bateau mouches' in Paris, this was a bit less exciting, but still worth the go as kids love boats and it gives you half an hour of calm where you don't need to walk and hull the kids around. I'd also certainly recommend it if you only have a short time in London as you get to see some of the main monuments in a short time.
adult 17£, child 8,50£
Free with London pass