>paying off my student loan when the interest rate is linked to inflation and is written off in 25 years (by which time most people will still be nowhere near clearing the debt)
>

I'd take a sabbatical from my job and travel the world for a bit. I've always liked to think that I'd invest some of my money in community projects that south Wales desperately needs, maybe give my old school enough money to finally buy some kind of sports equipment to replace the one 30-year old cricket bat that makes up the entire kit room. I'd then get a membership at Lord's and then put the rest of the money into my pension fund or something.