Skydiving. I hear how exhilarating it is and amazing that sense of freedom and being on top of the world. Unfortunately in my head it is a 50/50 chance of life or death and I don't like those odds.
The second is a little sad but I wish I had given myself the chance to be happy by being brave and shaking my life up massively when the perfect opportunity presented itself (although I've since had a baby and he is my world I would not change!).