As famed for its antiquities as its clutter, London’s British Museum has cleaned house — or, rather, yard. A glass-and-steel roof now caps its spiffed-up two-acre courtyard, christened the Great Court, creating a gleaming public piazza in the center of the city. Designed by architect Norman Foster, Europe’s largest covered square includes the famous rotunda Reading Room, along with new galleries and a restaurant. Best of all, it’s open late, until 11 p.m. Thursday through Saturday. Log on to www.thebritishmuseum.ac.uk.