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We visited famous historical sites.
Literal
Historical-famous-places [object-を] sightseeing did [polite past].
歴史上の ('historical,' literally 'upon-history') is a fixed modifier form. 名所 ('famous place, scenic spot, landmark') is a common compound — 名 ('name') + 所 ('place'). 見物 is a suru-noun meaning 'sightseeing, viewing attractions.' 見物する = 'to sightsee.'