Boy is that an essay question; I have two answers to that depending on how anyone would rate different things.
For me personally, the craziest once in a lifetime experience was my time spent in World of Warcraft. An 8 year long experience and I know that sounds kinda sad but hear me out: initially it was pretty good, first time committing a lot of hours to a game and feeling a strong drive to get better, gloating to schoolmates and the like, but eventually to get to the next level and set off on your own. Finding a good guild took time and it wasn't the best, just a collection of people wanting to play certain content with you.
But after years the guild becomes a really interesting family with such a variety of people - the were German guildmates who helped me with my German in school, a relationship formed between the guild leader and an officer, and when I had to go to London for work related training they offered to let me stay with them - I ended up staying in a £2M flat in the direct middle of London (the woman was a retired lawyer) and the woman's kids sort of knew me from hearing about me in the guild (they were about 8 and 10) and treated me like a big brother and idolised me which was super sweet. They had the absolute best behaved cats I've ever seen (you could feel them about to pull their claws out and restrain themselves) and sleeping on the floor in a sleeping bag and waking up in a pile of cats was great. They also treated me to a full steak dinner!
You realise that the whole guild is full of people from all over Europe - a narcoleptic swedish chef was a legitimate person and not a muppets character, there was a hardware engineer in Italy that loved my music playlist and still follows it to this day, a student from Amsterdam who met me and another local friend at the London zoo. A girl from cyprus who is the best gamer I've ever played with. And I was the one who main tanked and lead all these people to being the number one guild on the server through bosses that took literally months of 12+ hour weeks to take down.
Outside of the interwebs I suppose I went on a firefighter course (I'm 6'0" and 55kg, so not exactly prime fireman material) and in spite of the firemen initially not thinking much of me at all, managed to push myself through 3 weeks of drills and firefighting exercises - erecting, holding steady, and climbing up the very tall and heavy ladder, unrolling the fire hoses, pluging them into hydrants, operating the pump and holding them steady, rescue training with breathing apparatus in smoke filled maze-houses looking for dummies (child dummies can't half hide), abseiling off of a 6-floor tower, car rescues with the jaws of life, and actually being driven around in and driving a firetruck and operating the siren. It just feels really weird that I had access to all of that as a nerd who sits behind a computer all the time. (The fire engines were all automatic transmission weirdly! - I've only driven stick before or since)
What's the story behind the one thing you most regret not doing?