I’d set up a website where people could request money from me for anything. I would read each submission and decide whether I wanted to give them the money or not. I’d be entitled to free products/services if that money is used to start a business of some sort.
I’d rent out the Superdome for the world’s biggest Bumper Cars pavillion. Or maybe, who’s got the convention center with the largest single exhibition hall?
Paint the Statue of Liberty to the original copper.
Run a reality game show for sucking up to me: I put up a prize of several million, and the contestants have to be my lackeys and yes-people for a half season. Whoever is the best toady gets the prize. Then whoever was eliminated first gets to be the big man in the next running of the show.
Build a tree house in a giant sequoia.
Collect stock certificates and memorabilia from failed dot-coms and CEO-goofs. Open a museum to irrational exuberance.
Start an auto racing team, and disdain all commercial sponsorship.
Buy the naming rights to EVERY pro sports venue in the country and give them all REAL names. (Jack Kent Cook Stadium was good. FedEx Field, my ass!)
Start a company to build a pyramid. Start another company to tear down the pyramid. Have the companies sue each other.
Hire a pastry chef team to work at the local soup kitchen.
Bet the limit at roulette, on 18.
Heck, I can do that now! 
Not original, but…a guy in England built a trebuchet (catapult - sp?) and he throws cars around. That would be the cat’s pajamas.
I would construct a row of dominoes that started at the north pole and spiraled all the way down to the south pole. I would pay for whatever construction was necessary to make one continuous sprial of dominoes standing on end. When it was complete I would knock the first one down and then fly to the south pole to see how long it took for the last one to fall.
I would pay for a team of scinetists to design, build and program a Massively Distributed Network of say 1,000,000 P4s so I can truly say that I have the best/fastest computer in the world (would that be faster than a Kray3?).
Use that computer to find The End Of Pi.
Make a web site that has live updates of Pi (number by number), which also has a search function to find your name in Pi.
After Pi has been calculated to a couple trillion digits, hire a bunch of PHDs to “prove” that the Bible and LOTR is hidden in Pi.
Start a huge mystical kult based on that “discovery”.
Mass produce T-shirts which say (on the front) “When come back, bring Pi” and (on the back in huge letters) “Mmm I like Pi”. Give every Doper one of the t-shirts.
Create a massive army of bloodthirsty, combat trained, geneticly altered killer chiuauas and Try to Take Over the World.
I would pay beautiful women to be my clothing.
I see where you’re going with this, and I’m not going to let it happen. I’m not too proud to wear a dress.
As for me - hookers and blow for everyone!
Ya know, dave’s a luckier guy than we’ll ever know!
I would destroy LA, and have it rebuilt out of Lego.
Get an old man with a beard to take the fundies away, saying it’s the rapture. They’re happy - they get their rapture, we’re happy, no more fundies. It’s win-win.
Bridge the Atlantic Ocean.
Fill Lake Michigan with beer.
I’d play TheSims - using real people
I’d buy the bridge that Gex Gex just built and replace it with a giant roller coaster
Well first I would buy antartica and make the biggest luge track!
then I would buy the companies that the people that made fun of me in high school worked at then fire them and repeat this till they die.
then I would help fund BioHazard’s Pi adventure.
I would hire ever b-list entertainer and then put them in chains and chain them to the ground and then let poor kids from other countries the b-list entertainers tried to help, kick the entertainers ass.
I would make the minimum drinking/voting/smoking/everything age 17 because thats my age!
I would buy russia and make the land sacpe and everything just like the map of middle earth (lotr) and let people live there and all that what not!
Or I would pile up all my money in the middle of nowhere and litteraly Blow it up!