The legal tender in Grenada is the Eastern Caribbean Dollar. Like most currencies, its value against others moves for better or worse.  However unusually, its rate against the US dollar is at a fixed rate (by the eastern caribbean central bank) of 2.6882 EC to the US Dollar when buying EC.  

This means there is a slight anomaly in changing money.  The banks profit is its spread, or the difference in buying vs selling rates.  For most currencies, this gets them a healthy profit, but for EC vs US, they are restricted.  Therefore unusually, if you can get a GOOD rate in your own country for US dollars, you know the rate above, and you may well discover by changing money to US in your own country then exchanging that for EC in a bank in Grenada, it is possible to save a few percent.  This is possibly not of interest if you want a few hundred, but larger sums attract larger savings of course.

It is also worth knowing that in some countries it is possible to obtain perfect rate credit cards.  These will give you the real current interbank exchange rate, and charge you nothing for the transaction.  These usually are the best option if you can obtain one.  In the UK the Halifax Clarity card is an example. 

The US dollar is also widely accepted, but there is no fixed exchange rate for this outside banks, so you can expect to pay more for goods or services in shops, taxis etc.

Banks are habitually open from 8am to 3pm Monday to Thursday and 8am to 5pm on Fridays.  Queues are legendary, a real opportunity to meet locals...

ATM machines accept all major credit cards as well as local debit cards.

There is a currency exchange at the airport that charges a service fee, and there is an ATM, however this may not give the best results.  There are a number of banks with 24/7 machines on the way to Grand Anse, noteably at the airport road/main road roundabout, at Wall Street, and in the small mall opposite the Radisson.