I could go into way more detail over this, but I'll try and keep it short
First thing I would do would be to sink about 5-10 million dollars into a new house built from the ground up with everything automated, everything digital, and everything over the top ridiculously high tech, with a garage about the size of an airplane hangar. I would then run my ass to several dealerships:
Subaru for an impreza STI
Audi for an RS4 and a R8
Ford for a GT500 and possibly a GT40 (The one that came out in 2003)
Hummer for a goddam hummer
Lotus for an Exige S
Lamborghini for a Mercialago
Ferrari for a 599
and probably every other dealership with a car that interests me.
I would then move on to get hard, legal copies of everything I have every downloaded in my life, including all my music, video games, and anime. From there I would contract a designer to go ape shit making the house look as modern as possible, with perfect organization of all my recent media purchases. I would build an epic manga/comic/book/magazine collection on top of that.
After that is all said and done, I would do something very out of character for myself and splurge on a decent wardrobe filled with decent clothes and ridiculous watches, sunglasses, and shoes.
After making sure my all my collections were as epic as possible, I would go crazy on tigerdirect and newegg stockpiling everything new and computer related to build several kick ass computers and start overclocking like a madman.
Once the comps are done, I'd move on and spend about 300,000 dollars on a home theater.
To be honest I would probably live like a playboy, wrecking cars because I get bored of them, buying anything shiny, etc etc.
Somewhere along the line I'd pay off family loans (if any) and pretty much spoil the shit outta everyone. I would also, most definitely, contribute regularly to charities considering 300,000,000 bucks is alot of goddam money.
Other than that, I'd probably buy lots of random properties across the globe, the first purchase being what ever I can get my hands on in or around Akiba :p
Then I'd hire a financial advisor, invest, pretty much do what I can to ensure the wealth stays within my family for as long as possible.
That is alot of money, so theres not much limit as to what you could do. I'd pretty much, after I took care of most of my sick greed and materialism, just try and experience everything I possible could :p