If there are different regions and city-states at war with each other, it is realistic to make different currencies. We can make one town deal with currency A and the other with currency B and then have NPCs exchange both which can help create a merchant class.
Also more specifically (just to confuse the players and make it interesting)
Currency A:
1 gold = 100 silver = 1000 copper.
Then currency B can be:
1 corinthian gold = 100 corinthian silver = 1000 corinthian copper.
When you appraise the corinthian gold, you notice that it is made of 60% gold and 40% dirt, thus it has less intrinsic value than currency A.
Currency C can be:
Wheat, rice, salt in weight.
Because they are so poor and remote from the big city the small remote villages use food.