This piggybacks off the biggest thrill thread, but I want to draw a distinction here.
For this answer, it must be something that most of us could do given roughly $200 and a free trip to anywhere (relatively convenient) in the world. So it can’t be Everest and it can’t be piloting a chopper through heavy fire. It also can’t be something on private property or inaccessible for any reason.
It’s harder than it sounds to come up with something. Interesting things typically aren’t available to the masses, or else they are no longer interesting (I’m sure Paris is cool and all, but walking around taking pictures seems pretty generic)
The best thing that comes to mind for me was riding a bicycle down Pike’s Peak. I went down with guides and about 20-30 other riders. We stopped every mile or two so that the slowest riders could catch up with the fastest riders. I would estimate that it was approachable for moderately healthy people age 12-65. It was actually much safer than it sounds. It could be rode at your own pace, whether that be 5mph or 45mph.
Whitewater rafting is fun as well, but I’d say that the strain to do that was far greater than biking downhill. That would be a limiting factor for many.