Night at the Museum (...actually a day) in London
I spent a weekend in Knightsbridge recently and was next door to both the London Natural History Museum and the Victoria & Albert Museum. Some cities have an occasional "free museum" day, in London it's free museum day everyday. I took advantage by visiting both and had an incredible time.
London's Natural History Museum
Victoria and Albert Museum
At the V&A museum, I took the opportunity to visit the Balenciaga exhibit as well.