This came about because a poster on the "what did your baby give it's older sibling" thread mentioned the Fire Engine Museum in Sheffield. I'd never heard of it but I now know where we'll be going for the day when DS turns 3 in August! I can't believe I didn't know about it.
My contribution would be the Coventry Transport Museum because it's free to get in. It's another place I'd never heard of until we made a concerted effort to find places to visit to with DS.