Pay debt, quit job, take time to do something worthwhile, learn worthwhile things, go travelling and see worthwhile things. Record music.
Pay off parents' debt (they're probably close to not having any).
I'd watch with curiosity how it affected my relationships. If anybody suddenly wanted more involvement where they'd had little before they'd be getting short thrift. I'm not vindictive by nature, but I think someone trying to cosy up and take advantage of me could cause me to be incredibly nasty indeed. The social effects would be the most interesting to experience IMO.
I've read my vague first sentence back and I'm tempted to ask myself why I can't do any of that stuff now. A more youthful, optimistic version of myself would probably give it a go. In the real world, time and energy is limited to some extent by life and basic survival getting in the way. They say money doesn't make people happy, but it definitely makes it much easier for people to be happy if they want to be. No doubt about it.