Just a short stroll across the Millennium Bridge (which sadly no longer wobbles) from the Tate Modern or a wander up from Trafalgar Sq. The dome is the third largest in the world, designed by Christopher Wren in 1675 after the great fire of London destroyed the original cathedral. The Whispering Gallery inside the dome offers a lesson in acoustics while the view outside across the Thames is excellent. Open 8.30am-4pm, Mon-Sat, admission £9.50, £8.50 students. Closed for religious observances and Sundays.