This sounds dumb - but since I'm not dealing with any large sums of coins, I've mostly been investing mine in Magic the Gathering cards. Thanks, ABU Games!

The game is still going strong after around 20 years, and I'm finding a ton of the cards I had ten years ago have practically quintupled in value. Worst case, I've got some shiny pieces of cardboard to play around with that I can sell off for a new PC in a few years.