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The Serpentine is the huge man-made lake that splits Hyde Park and Kensington Gardens in half, and if you're feeling energetic you can spend an hour pedalling around in a little boat.
It's a lot more fun if you just sit back and enjoy the sunshine so if you can convince someone to do all the pedalling that's perfect.
If you don't fancy the exercise then you can try the SolarShuttle which glides silently across the water powered by the solar panels on its roof. The SolarShuttle carries up to 40 people and has a couple of extra wheels so they can let the children pretend to be Captain for the trip.
Note: Dogs and children under 12 months are not allowed on the pedal boats. Children under 12 must be accompanied by an adult.
There are plenty of things to see in the park once you've finished on the boat. If it's a Sunday lunchtime then you could go and visit Speakers' Corner (there's not much point going on any other day of the week, because there won't be any speeches). You can also go and see the Albert Memorial and Princess Diana Memorial Fountain. Or you could walk across Kensington Gardens to Kensington Palace and see where Queen Victoria and Princess Diana lived.