The Victoria and Albert Museum is the world's largest museum devoted to decorative arts and design, from Europe, North America, Asia, and Africa. It contains examples of ceramics, glass, costumes, medieval objects, sculptures, prints, etc., dating back as far as 5,000 years. You could spend a day there and not even begin to scratch the surface of interesting things to see. Definitely a good admission price being it's free, and much less crowded than the Natural History Museum right next door.