Jumping off a five metre diving board.
Abseiling down a cliff, getting half way down and sitting down against the rock face (feet pointing up the cliff), then laying down (so head pointing downwards), then letting go of the rope and stretching my arms out straight...actually, that last part was what I was SUPPOSED to do. I don't actually remember if I actually got into the full position or if I couldn't quite make myself let go of the rope!
Walking the length of an Israeli Wadi along an unfenced a goat track that ran roughly thirty or forty feet up the wadi wall with a sheer drop to the wadi floor. (It was probably a trek of about three or four miles, all told. THEN, of course, we had to climb OUT of the wadi to meet the coach...)
None of those sound terribly impressive, until you realise that I have absolutely NO head for heights. The first one was the only time I've ever let anyone talk me into doing something I didn't want to do (I was about twelve at the time and didn't have the spine to say "Not on your life!"); the second was on an adventure holiday and actually wasn't too bad because most of the time you're not actually looking down at all...letting go of the rope though...not so much!; the third one was definitely the worst, though. I was an absolute wreck by the time we got to the coach, and I got no sympathy of anyone because "We're all tired", no, I'm not "tired" I'm "terrified". There's a difference.
Then there was the school talent show where I sang a Madonna song, dressed in a VERY low-cut top, very VERY short mini-skirt and, effectively, propositioned my head mistress...