If you're already looking up a country, fetching its currency shouldn't take another request just to learn the code or display its name.
These fields come back with GET /country/US:
{
"country": "US",
"name": "United States",
"currency": "USD",
"currency_name": "United States Dollar",
"currency_symbol": "$"
}
That gives a country details page enough to say which currency is used. Keep the code when displaying amounts across countries, though. A dollar sign by itself doesn't tell someone which dollar you mean.
When you need currency-specific details, follow USD to /currency. /currency/USD includes decimal places; add ?deep=true for the countries that use it, included on every plan. To get a reference exchange rate, use a pair such as /currency/USD/EUR.
The distinction keeps a country lookup useful without stuffing the entire currency record into it. It also works when several countries share a currency: their country records point to the same currency code, so your application can resolve that currency once and reuse the result.