The London Science Museum is free* and had a ton of hands on exhibits for kids. We planned to do the zoo, but we let the kids sleep in and didn't have a lot of time before the wedding rehearsal.
*they have a suggested donation.
Dancing with the thermal cameras.
Yes, she is.
There was a room showing the evolution of things like washing machines and toilets, and how they work and letting you try to run it manually. And they had the first computer game ever (Pong) on display.