Until next Sunday, the Science Museum is running rides on a replica of Stephenson's Rocket up and down Kensington Gardens. It's a fiver per adult and free for under 5s but you get a badge, a pen, a pencil, a notebook, a magnet and two postcards as well as a ride up and down the track. The driver explains how the engine works and throws coal into the fire and tests the pressure. My DC loved it.
I've always thought the Science Museum would be a bit too old for my DC but there is the Garden in the basement which is aimed at 3-6 year olds which was brilliant. There are lots of places you can eat a packed lunch and loads of other things to do and see too. And it's all free!
Highly recommend