A question for the economist here, does paying my country's national debt help?

For the low-hanging fruit answer: Invest it. Too many things to do for 300 billion to be enough.

I'd make a development fund and fund renewable tech, green startups, lobby politicians for policy changes like a carbon tax to pay for social programs.

I think it's best to use the fund more or less like Bill Gates did.

I'm going to fund and build a ton of mass transit projects.

300 billion, secure myself first, then build up companies in my interest, then use that capital to influence politics, then secure policies that will help my personal and business objectives, then create a charity that will have a tax exemption for planting trees and r&d a biodegradable, easy to manufacture, durable, plastic that will replace all current non-biodegrable plastics. Wouldn't become prime minster because you can get the most done behind closed doors. Also need to use some of the money to secure my kids future.

Retire to some island in the middle of nowhere and build myself an evil villain lair. Once money runs out threaten for 1 Beeeellion dollars. If 300 billion usd isn't spent then donate the rest to some child cancer hospital or something anonymously post-mortem.

Any other choice would be suboptimal.

300billion of what?

Create the world's largest charitable foundation and make everyone beg before their presentations.

Donate to cancer research

Eradicate cars, have reliable public transit that runs 24/7 and departs every 5 minutes. All deliveries are via helicopter for businesses and drones for personal orders. Turn parking lots into something actually useful. Make all fast food places completely automated and open 24 hours. Eradicate homelessness and poverty.

burn em