Aurora borealis on polar routes
Sun dogs above the plane
Sun glories below the plane
Space shuttle launch
Fourth of July Fireworks on a flight across the Midwest
Tracer rounds in Africa
Great wall of China
Grand Canyon in NW Arizona
Canyon De Chelly in NE Arizona
Oil tankers at sea in the Pacific
Other aircraft above and below our flight level
Full moon above the clouds
Mexican pyramids near Mexico City

I look forward to the 787, which has even larger windows for gawping out of.