What's the must-see thing in London?

The Tower sits at the eastern edge of the City, right where the old Roman wall met the river. You'll smell the damp stone before you see the ravens — that cold, mineral scent that seeps out of walls built in 1066. The Crown Jewels are the headliner, and they deserve it: the Imperial State Crown alone carries 2,868 diamonds, and the moving walkway past the display cases means you get roughly ninety seconds of viewing time during peak hours. Go at opening — 9am Tuesday through Saturday, 10am Sunday and Monday — and walk straight to the Jewel House before doubling back to the White Tower. The Yeoman Warder tours start every 30 minutes from the main entrance and run about an hour. They're good; these are retired military with a talent for gallows humour about the executions that happened on Tower Green. Admission is currently £33.60 for adults, £16.80 for children, and you should buy timed tickets online at least two days ahead. The nearest Tube is Tower Hill on the District and Circle lines — you'll exit the station and the fortress is right there across the moat.

The British Museum in Bloomsbury is free and open daily, which makes it the single best rainy-afternoon option in the city — and it will rain. The Great Court's glass roof filters grey London light into something almost warm, and the Reading Room dome at its centre feels like standing inside a marble bell. Head straight for Room 4 (the Rosetta Stone), then Room 18 (the Elgin Marbles — still controversial, still extraordinary), then the Egyptian galleries on the upper floor. That's ninety minutes well spent. You could stay all day, but the building tires you out before the collection does; the stone floors are hard on feet and the café queues get brutal after noon. The museum sits a ten-minute walk from Tottenham Court Road or Holborn stations. No booking needed for general admission, though major exhibitions charge £15–20 and sell out weeks ahead.

The Westminster stretch along the Thames is the postcard, and it earns it. Start at Westminster station — you come up the escalator and Big Ben's clock face is just there, filling the exit frame at an angle that still catches Londoners off guard. The Elizabeth Tower's bells sound different in person than on television; the low G-sharp bourdon note of Big Ben proper vibrates in your chest at close range. Westminster Abbey sits across the road. Entry is £27 and takes about ninety minutes; the Poets' Corner tombs alone — Chaucer, Dickens, Hardy, the lot — justify the price if you care about English literature at all. Walk south along the embankment toward Lambeth Bridge for the best photograph of Parliament from the river. Mind you, if you're only in London for two days and forcing a choice between Westminster Abbey and the Tower, take the Tower. The Abbey is a church with famous graves; the Tower is a fortress where the graves were made.

Skip the London Eye unless you're travelling with children under ten who need a contained activity. The queue runs 45 minutes to an hour even with pre-booked tickets, the capsule rotation takes 30 minutes, and the view — while wide — is better from the free Sky Garden at 20 Fenchurch Street, which also has a bar. Madame Tussauds is overpriced at £37 and underwhelming past age twelve. Buckingham Palace is worth seeing from the outside during the Changing of the Guard at 11am on ceremony days — check the schedule, it's not daily — but the interior tour runs only in summer and the rooms feel like a hotel conference centre compared to the Tower's raw medieval weight.