One option is numismatically significant gold and silver coins.
Why pay the premium? It assumes there'll be someone around later on with the capital and the interest to repay the premium you paid. Bullion coins would be better -- "junk" silver for small considerations, and a range of bullion-coin weights from small to large.
British sovereigns struck by Good Queen Vickie and her heirs don't have numismatic premiums attached and trade at the same sort of spread as K-rand and Maple Leaves; they're a little larger than a U.S. half-eagle, and weigh a little more than a quarter-ounce bullion coin. Without parsing values closely, that'd be the range of coin sizes that would be most useful in future, assuming somebody cancelled the future.
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