You're right. He seems to do personally challenging things which are relatively, only relatively, inexpensive. It doesn't sound as if being just a passenger on an expensive, but brief, mission would do it.

He doesn't seem like the Evel Knevel type.

A Soyuz-ISS ride would cost more than he usually spends, and doesn't require endurance, but I wouldn't put that past him if he had the money. I think he'd do it for the experience rather than the challenge.

The Iditarod isn't a unique event, and he did that. But that was earlier in his history. While he seems to have graduated to unique events, he might go for something dramatic other folks have done. Everest is still unusual, the 7 summits still more unusual, not as newsworthy as they used to be.

We'll see. My guess is that whatever he does, it will get publicity.

Arf

Otherwise, there is the one-person 'round-the-world non-stop airplane flight, I think. There are all manner of rivers with huge rapids to run, and his money could purchase design and build for some novel boats for doing that.

For endurance, a one-person float from the headwaters of the Amazon to the mouth, or maybe the Nile comes to mind.

Or,