The London Transport Museum is far broader than a collection of old buses. It tells the story of how London grew around its transport systems, from horse buses and early underground railways to Tube design, maps, posters, and the red double-decker bus.
The museum is especially good with children, because many displays are built around vehicles you can climb into or look through. It also works for design-minded adults: the roundel, Tube map, and poster collection are part of London's visual identity.