On a recent trip to London, UK, I took the kids to the London Science Museum. It's an excellent free museum with some superb exhibits related to different areas of science, ranging from astronomy to zoology. It's all free, and is a great place to spend a rainy afternoon.
There were a couple of neat exhibits related to space sciences. Firstly, they had the mirror belonging to the Great Rosse Telescope on display. While I just knew that I could sneak it out of the musem without issue, there would have been serious problems on the flight back home!
Additionally, they had the command module/capsule from the Apollo 17 flight on display. It was brilliant to see this, but the astronauts involved must have been midgets as this capsule was very small indeed!
In any case, the London Science Museum is one of the many, brilliant museums in London and I highly recommend it.