I think it would be more pertinent to the analysis if the question was "What, as an adult, would you have liked to have done but never had the opportunity?"
I have never had my ears pierced because I have not wanted to. I could very easily do so, but I don't want to.
I have never had a one night stand. I've had dozens and more opportunities but it's a situation I have managed to avoid. It's not what I want for myself.
I have never had a tattoo because I don't want one.
I have never been to a baby shower. I have been invited to two but I declined.
Genuine reasons, but I didn't want to go anyway.
I have never taken illegal drugs of any kind. Again, lots of opportunity but not what I wanted to do.
There's loads of things I've read above that I "have" done though.
Ridden a horse. Camped. Been to the USA every year for the past decade.
Holidayed in the Caribbean. Been a bridesmaid a mind blowing 12 times.
Played Bingo and once won £400. That was loads of money, to me. I've smoked in the past, and ridden bicycles, and done a bloody load of stuff that previous posters apparently haven't. I've also looked after my profoundly disabled daughter for 30 years which is an efficient barrier to doing stuff you'd really like to do. Which is why I think a differential should be made.
Things you have never done. Or things you have never done that you would have like to be able to do.