The Bradbury Science Museum is the public visitor centre for Los Alamos National Laboratory, and it is free. The displays cover the history of the Manhattan Project in unusual depth: the recruitment of scientists from universities across America, the construction of a secret city on a remote New Mexico plateau, and the development of the two bomb designs (Little Boy and Fat Man) that ended the Second World War.
Artefacts include replicas of the two bomb designs, historical photographs and documents, and exhibits on the modern laboratory's work in nuclear weapons, national security, and energy research. The museum is not sensationalised — it presents the science and the decisions plainly, and leaves judgements to the visitor. Allow an hour and a half.