Just how are you going to stop me from investing? I could buy hundreds of things that gave me back more money later.
Anyway, I would pay off my student loans, and get a bigger apartment. Then i'd get my girlfriend to live with me, buy some really nice home theater equipments and other hi-fi articles, upgrade my PC stuff again, and buy all the stuff I would need in my apartment.
After all this, there would still be tons of money left. I'd buy some stuffs for my girlfriend, and probably give some money to my parents and close family/friends, then invest the rest, because doing anything else with them would be plain stupid.