The Crystal

Although at times it does come over like an expensive advertisement for technology firm Siemens – whose groundbreaking sustainable building it inhabits – the Crystal is now City Hall, headquarters of the Greater London Authority and the Mayor of London; the sustainability exhibition has closed, but parts of the building are open to the public free of charge and there's still a cafe, with views over Royal Victoria Dock. You can tie it in with a London Cable Car journey across the river to North Greenwich.