A second harem.
An asteroid. I’d put up a $5 billion bounty to the first company or nation that can move an asteroid of at least 100 meter diameter into either earth or lunar orbit, extract useful quantities of ore, and extract water ice from it.
Chantilly.
Forest, palace, lake and village. I’d let the current neigbors stay where they live, no problem, and build a new road around the side of the palaces’ grounds where the newer palace is (the one housing the Museum of Horses); I think the current road may be the original.
I’d live in the Renaissance palace bordering the lake (the one with the blue roof) and build an addition, looking sort of like a weathered barn so it wouldn’t stick out like a sore thumb, on one of the other sides of its quadrangle. That would have a gaming area, a Serious Computers Room of Doom and dorms. The SCRD would be open for all kinds of non-profit stuff, from students working on their thesis to astronomers or biochemists needing a bigger computer for some specific work. The bathrooms and bedrooms would be to kill for; I’ve been in a little hotel nearby whose owners I hope I’d be able to convince help me decorate 
anonymity
and a harem with a sexy pair of women from every ethnic stereotype on the planet
There is a guy called Billy the Fridge who supposedly has sponsorship from billionaire Alki David to go around doing stunts. If I were a billionaire, I think I could get behind that, paying some guy to perform pranks.
Here is a NSFW article and video of Billy the Fridge scaring the hell out of a member of the Westboro Baptist Church. If I were a billionaire, paying people to do stuff like that would be fun.
I would buy the ex-TEAL Short Solent that’s hangared up at MOTAT, pay to get it airworthy, and then just hire a pilot and crew to fly me around the world in my private flying boat, hopping from Auckland to Sydney to Darwin to Singapore and around the Pacific like in its heyday. Or if MOTAT won’t part with their Solent, they have a Sunderland they’re not using.
California. All of it.
I would buy every existing Frank Lloyd Wright designed structure in existence and destroy them one by one on international TV in a different way every week. The first few in the series will just be simple burning, wrecking balls and implosions but then things will get more interesting when we move to the more significant buildings. The last two to go would be the Guggenheim and Falling Water. The Guggenheim (not the works - just the building itself) will be attacked by a huge gang of starving artists and architecture students for hire until there is nothing left but a pile of rubble. Falling water will be destroyed by an artificial flood of Biblical proportions that causes the river it sits on top of like a fat lady on a toilet to rise up and suck it right back down.
Be a movie director, film movies with $500 million budgets each.
I want a sea-worthy, modest, but big enough to live and travel in for extended periods, houseboat(maybe a fix-up job) and a shack/small house just by the riverside, with enough room for sleep, work, cook, eat and unmentionables. It needs to be fairly close to an urbanish city. It would be nice with some outdoor space too, maybe, and a drivers license for boat and car.
I like the way you think – particularly the live-in sushi chef! But please, drop the high school idea and consider establishing a proper university for these dudes to hang out in – it should have a large beautifully landscaped campus and all the buildings must be in the collegiate gothic style.
For my part, I’m pretty much OK as I am, except maybe a nice vehicle to get to places I’d like to go, one that looks as great on the outside as it does on the inside. A nice English country house with some character and tradition. This one is nice, too – it appears to be occupied, but I’m fine with both pictured occupants staying*. And an English country house is not an English country house without the mandatory pond. That, and the aformentioned mega wealth, is all I ask, and thank you.
- ETA: Since I’m paying for the place, I reserve the right to throw out either one if either turns out to be a bitch.
If money’s no object…
[ol]
[li]Some rural properties in various locations around the world[/li][li]As described upthread, a Mazda Miata. I’ve always had a soft spot for them.[/li][li]Have a TSR-2 built and reconfigured aft to carry a few passengers in luxury, and a crew on standby[/li][li]A chunk of land, maybe 5 km by 2 km, on which I would design and build my own town.[/li][li]an island somewhere in Canada[/li][/ol]Condos or apartments in different cities around the world.
Land. Land that needs to be preserved, sort of like how Teddy Roosevelt did with national parks. For myself, I’d like a large plot of nice land with a pristine viewshed and a regular house with all mod cons. Oh, and a state-of-the-art studio.
I thought Teddy’s idea was to keep all that beautiful land out of the hands of whimsical billionaire assholes like us? 
I’d buy the rights to Science Fiction and Fantasy. Yes, you heard that right - all the existing IP, and any new IP as it comes out, by whatever means possible, including blackmail and bribery - anything short of violence. Also the major studio and TV outlets for SF&F. I’d allow any written material to be published freely, but visual and games media versions only get made on my say-so. If I’ve seen it (or enough of it to hold an opinion, or possibly I’d allow a small coterie I trust to do approvals), and it’s OK, only then does it get released.
I will obtain as much existing SF&F IP as I can, by means fair or foul (short of violence or threat of violence). I will buy up as much new IP coming out as I can. Also, greenlight-level interest in as many visual media outlets as I can (TV channels, studios, games developers).
Then, the only way most new SF&F movies and series get made is if I, or one of my coterie of trusted approvers, thinks it’s a good idea. And we’ll reserve final release approval too. There will be no more Sharknados in my world. Also, no Peter Jackson Silmarillion. And whitewashy shit like Gods Of Egypt is right out. Expect SF&F movies that can easily pass the Bechdel test, that reflects a strong pro-Left, -feminist, -diversity, -social justice agenda, with diversity of cast previously only the domain of BBC and Canadian productions - basically, the nightmares of every Sick and Rabid Puppy made manifest in full-on multimedia form.
But don’t worry, I’m not highbrow - Transformers 6 will still get approval 
Why not get a brand new jet seaboat and instead turn that into your flying palace. Theres a number of options to choose from like the Beriev BE200.
Or for a modern prop seaplane there is the Shinmaywa US-2.
Our “megapurchase” would be a new home. Nothing big, but definitely customized.
I’d get some games made. One idea in particular would involve commissioning a line of action figures for a 3.75" scale board game.
Purchase #1
A massive stone castle constructed according to the Medieval practices, a la Guédelon Castle.
Purchase #2
Hire some sort of power of attorney-type person who can somehow keep me from turning into a spite-filled megalomaniac. I have a bad feeling that I’d have a lot of (literal) skeletons in my (figurative) closet if I had basically unlimited funds.
Purchase #3
1 example of every motorcycle ever made, maintained by a army of machinists, mechanics, cosmetic-type dudes.