Once in a lifetime trip

First things first, I hate this phrase. Quite simply because many years ago, I had the privilege of dealing with complaints for an airline, and this phrase was used very often by complainers to dramatise their issues. And often, it was used by people who had flown to Orlando, and when you actually checked their flight history, they had done the exact same trip 10 times, all within the last 5 years :D

But what would your ultimate 'once in a lifetime holiday' be?

I would love to follow the England cricket team around Australia for The Ashes. I would also quite like to drive across the US, doing all 48 contiguous states in one trip.