High School
Once was great but there ain’t enough money in the world to get me to go through that again.
Visiting China. A truly moving, breathtaking, exotic, eye-opening experience which also sparked the most interesting discussions I’ve had with my parents in my entire life. All of which made the abysmal restroom facilities, soul-numbingly monotonous food, and almost total lack of fun activities tolerable. Could not be like that ever again.
Seeing Titanic in a theater with my family. Honestly, I don’t know how the hell we pulled it off, but we had a blast, and my dad has rarely been more engaging than when he discussed the movie with us afterward (he compared it to West Side Story). An amazing long shot that any of us would be lunatics to try again. (This was the last time I ever watched a movie in a theater, BTW).
Watching Grave of the Fireflies. An intensely emotional and moving tale that does NOT benefit from extensive overanalysis (a point forcefully driven home by the rambling commentary on the DVD and some particularly ludicrous IMDB reviews).
Most of the PC games i used to love have not aged well at all. Playing them now makes me a sad panda, since what was once marvelously exciting is now hopelessly archaic.
Hint… its not just graphics that have improved with time. Did you know that there is no strafe in Wolf3d? How can you have a fps without strafe?! There are a thousand little annoyances that experience has taught developers to avoid.
Riding an elephant through the Thai jungle in a rainstorm clutching a purple umbrella. Worth doing, not worth doing twice.
My first glimpse of Victoria Falls stabbed me right through the heart. Brought tears to my eyes, even.
Getting married. Beautiful experience. I can’t imagine how a second time could be quite as magical.
They haven’t. I caught some of the old X-Men cartoons I used to watch as a kid and they are really not as good as I remember them being.
Something I did see again that I wish I didn’t: Titan A.E. Saw it as a young kid and loved it. Watched it more recently and the nostalgic value was ruined because it’s a pretty poorly-done movie. 
For some reason though the original Loony Tunes just seem to get better. I haven’t seen them on TV in a while though which is a shame.