Things you only do once

[DedalusBloom]

Mine was walking down stairs with my hands in my pockets. Didn't end well

Camping
Pontins
Karaoke
Anal
LSD
A decade of celibacy within a stale relationship. All those years wasted in my prime.
Sex on the beach, at night in a car. Car got stuck in the sand. (So funny looking back, but an hour of panic for a 10 minute uncomfortable shag, was mortifying at the time!)

another vote for andouillette (shudder)
and I've eaten lots of 'interesting' things.

But I have to tell you some things I am just not even ever going to try:
bungee jumping
eating insects/worms

Water slides. Split my head open and got black eye after somersaulting mid ride. Horrific

Licked an ice cube tray for a dare