The Guyanese dollar (G$) is stable and pegged to the US dollar, which is widely accepted. You also may be able to spend your euros or even British pounds. Credit cards are accepted at Georgetown's better hotels and restaurants (usually for a 5% service charge), although not at gas stations or most anywhere else. Scotiabank is the easiest place to get cash advances, and its ATMs are the only ones that accept foreign cards.
Cash can be exchanged at banks, but cambios (foreign-exchange offices) offer better rates and less red tape. Sometimes hotels change cash for a small commission.