St. Paul's Cathedral

You walk up Ludgate Hill and suddenly there it is - that massive dome rising above the City's glass towers like a ship's hull, entirely incongruous and absolutely arresting. St. Paul's is still London's most recognisable building, even if the skyline has done everything possible to overshadow it. Inside, the scale is genuinely breathtaking. The whispering gallery runs around the base of the dome - you can actually hear someone across the cathedral if you speak into the stone - and climbing the 528 steps gets you proper views across London, though frankly your legs will regret it. The crypt holds memorials to Wren, Wellington, and Nelson, which adds a weight of history that doesn't feel forced. The cathedral draws colossal crowds, particularly mid-morning, so getting there early or late afternoon helps considerably. You'll need at least two hours if you're doing the galleries; an hour suffices for the main floor. Entry isn't free, which rankles a bit, but it's a functioning church as well as a tourist site. Families with young kids might find the crowds exhausting, though older children who like heights will enjoy the gallery climb. If you're staying at Lovat Parks near Chigwell - just a few miles out - St. Paul's makes an easy half-day in town. Combine it with the nearby Museum of London or a wander around Cheapside's Georgian side streets.