I've flown out of SEA probably 50 times. Once, on a UA 777 a few years back, we flew so close to the summit of Mt. Rainier that it almost felt like you could reach out and touch it. The pilot announced it in advance and I happened to have the perfect window seat for it that day. If there had been climbers on the summit, I imagine they would have been freaked out by our presence.

I've flown that direction on takeoff quite a few other times and always look - in fact, after that I always made sure I had an "A" seat out of Seattle. But we never got that close again... Seeing Rainier from a distance on either approach or departure isn't that odd: it was just that one flight that one day that was unusual.

Others:
- Grand Canyon a bunch of times
- Giza pyramids
- Lots of other airplanes in holding patterns seemingly right on top of us - common at LHR
- A few times, a plane cruising the opposite direction as us - always fun to see
- Hurricane Dennis from a few years back...we were flying in perfectly clear weather but you could see the storm in the distance, perhaps 40 or 50 miles away
- Lots of big thunderstorms in the summer in the Midwest. Always fun to see, as long as there's a way to get around them.
- Vegas at night. Can't miss it...
- A few nighttime baseball/football games. Obviously this isn't too rare out of LGA, but I've also seen a night game at Wrigley from the air before.