You can keep it there and withdraw it after 65 penalty free (have to pay taxes like a traditional IRA) or you can pull it out now and pay taxes plus a 10% penalty.

Probably better to withdraw and eat the penalty then, inflation will make it worth peanuts by the time I'm 65.

You can invest it in US or international mutual funds that historically have beaten inflation by 7% per annum.

If it's over a certain amount you can treat it like an IRA, otherwise you should get regular savings account interest, yeah.. I'm laughing too.

Fidelity has an HSA with zero fees that lets you invest just like an IRA. I don’t know if you can open one without being a US resident, however.

you can just withdraw it. Their website should have a way to do a bank transfer or write you a check. You will owe taxes, but if you're never coming back to the US, then whatever. If you're coming back, then you can generally invest your HSA in a mutual fund(most have some limited investment options accessible via their website) and then your HSA basically turns into an IRA retirement fund at age 65.