I don't ask this question to provide a "right answer" that will apply to everyone in the ultra-high-net-worth category. Instead, I'd like to argue that investing is not just a way of making more money for more money's sake. The areas you provide capital to, and the way you provide that capital, matters.
What you choose is ultimately up to you, but I posit that once you've surpassed that point of "enough," the satisfaction you gain from investing will be more about the impact of your money than the growth of your money.
Here are a few categories to consider:
Philanthropy
Business
Art and Collectibles
Legacy
Politics and Government
Startups
Impact Through Philanthropy:
Philanthropy is not an "investment" per se, because you don't get your money back in a direct sense. However, you can bring an investing framework to giving money away, and optimize for the type and magnitude of impact you want to have. And, you can multiply the wealth of the world (even if you don't get to keep it).
The trouble with this is that it can be hard to quantify the positive economic impact of your donations. While many charities do not provide these figures, The Gates Foundation has done exactly that. In their 2017 annual letter, Bill and Melinda claimed every $1 deployed by their charity yielded $44 in economic benefits. That dwarfs the best imaginable financial return from investing. I suspect that Bill and Melinda find far greater satisfaction from their efforts here than they would from turning their large pile of wealth into a larger pile of wealth.
Impact Through Business:
Some of the most impactful business people of the last century have left all of their chips on the table, concentrating the vast majority of their wealth in businesses they control. In recent memory, you have Steve Jobs at Apple, Bezos at Amazon, and Elon Musk at Tesla and SpaceX (among others). These individuals decided their biggest opportunities to influence the world could be brought about through their businesses, and as a result, have facilitated dramatic economic and technological developments. While you could debate whether their impact has been positive or not (I believe it has been mostly positive, on balance), they've undeniably increased the resourcefulness of the world by introducing and distributing cost-saving, democratizing technological developments.
Apple, for instance, produces tools that make it easier for more people to contribute creatively to the world, enabling endeavors that otherwise might not have been viable. Amazon's platform makes it easier for small businesses and start-ups to get to market, particularly through AWS which provides a vastly lower-cost, more scalable option for new software companies. Tesla presumes to benefit the environment by lowering carbon emissions, as well as increasing the proportion of time that cars spend in use vs sitting in a garage. SpaceX makes it cheaper to get to space, which will enable satellite-dependent technologies and businesses. And in the long run, it may also make it possible to shift industrial production off-earth, where mining and manufacturing could have fewer environmental consequences.
These founders feel proud of the impact they've had on the world. For the most part, they've kept their wealth concentrated in these enterprises because this is the way for them to have the greatest impact on the world.
This can also be true for you, even if you're running a small, local business. The people you hire, the opportunities you create for them, the difference that you make in the lives of your customers....These are important, no matter the size of your business. So rather than sell everything and donate the money to charity, it might be the case that continuing to grow your business might be the very best thing you can do for your community.
Impact Through Art and Collectibles, Influencing Culture:
There are a couple possible routes to this. One is to collect things that you appreciate, just for your own enjoyment. Buying a piece of art and putting it on your wall is a good example. Beyond your pleasure from viewing the piece, you might expect some of your collection to gain value over time, similar to an investment portfolio. Making money, in this case, is a happy accident that offers a nice hedge on inflation and other currency risks.
The second approach is to actively purchase art that you think will appreciate over time. Maybe you seek out new and emerging artists to buy their work. Maybe you even act as their patron, financing their lifestyle and their creation of these works, while assembling a portfolio of art for yourself. The search for these artists, and your interactions with them, can be rewarding, as can the feeling of having contributed to the creation of these works. This is a riskier investment strategy, akin to investing in tech startups. But like venture capital, you're directly supporting an endeavor that might not otherwise exist.
There's personal satisfaction to be had in using your investment capital to enhance the beauty of the world, while still earning a decent return. I personally think we could do a better job of this in American society. Most of us could learn a thing or two from visiting a city like Paris, where it feels as though everything is designed to be as beautiful as possible. Both approaches to collecting art promote this higher purpose.
Financial Legacy for Heirs:
This differs significantly from the previous categories because it means handing over control of your resources to people you trust to make good decisions with it. I think there are good ways and bad ways to do this.
A bad way would be to give it directly to your kids, outright, without preparing them to handle it. Doing so can stunt their ambitions, lead to bad (potentially fatal) decisions, and rob them of the confidence that comes from providing for themselves.
A good way is to raise your kids with good values, good work ethic, and to teach them how to handle money responsibly. Buffett says, "I want my kids to have enough money to do anything, but not so much that they can do nothing."
Another good way is to create an enduring charitable foundation where future generations can decide for themselves how to direct resources for continued, positive impact. This is potentially a rich experience that can help families stay connected around a shared purpose.
Political Pursuits:
This may or may not mean running for office, financing a campaign, or covering your cost of living when working in a low-paid or unpaid position. You might just contribute money towards whatever type of political reform you'd like to see. Lobbying your government can be a high-leverage activity for creating the change you want.
Impact Investing:
The final option, as I see it, is to provide capital to businesses who are doing things you deem worthwhile. It can be tremendously satisfying to contribute to the success of the next generation of entrepreneurs, and participate in the businesses they create.
All of this is just food for thought. Obviously I'd like for you to acquire $100 million and face this decision for real. But for now, what are the things I've mentioned so far that stand out to you? Is the philanthropy angle interesting? Would you rather be a titan of industry, even a local one? Do you want to set up an ongoing gift that outlasts your own lifetime, or would you rather see that money put to use while you can see its impact?
I encourage you to take some time and reflect on this, because your answers may offer some clues to what you should be doing with your money here and now.