Stonehenge (Salisbury, UK)
I know it's not a museum!
Stonehenge embodies many of the same properties I appreciate about museums. The palpable presence of history. The power to shape myth, their persistence over centuries (millennia really), their sheer strength... there's something ancient and undeniably 'human' in the energy there.
It's primarily appreciated as an archeological site and it seems every few years they're making discoveries, either at the site itself or in the surrounding area, which change our understanding of the history of Stonehenge. It's past is constantly being rewritten. As soon as one History Channel special concludes, a Discovery Channel special commences with 'startling new evidence' never before seen.
I had fun visiting with my wife and her parents when we lived in London. It's quite a ways outside London but easily do-able as a daytrip.