5E - What to Spend 100 Gold Pieces on

Alchemist's supplies, a vial of acid and an orb arcane focus
A suit of studded leather, passage on a ship for 250 miles and a halberd
Invest in 10,000 copper coins to use as small change or make coin mountains
Hire 50 mercenaries (skilled hirelings) for trouble-free day in town
Throw a banquet for ten people