Even though you are in a foreign country you will find that the US$ is accepted almost everywhere.
Canadians should get US$ before they leave unless they like giving money away as the exhange rate you get will be low and you will lose money.
There is usually a Bank Exchange at the hotel(some do it directly at front desk) and you can exchange your money there for Dominican pesos, and cash traveller’s cheques. Note they will only change traveller’s cheques into pesos. You need ID (usually passport) to cash travellers cheques. You cannnot exchange U.S. traveller’s cheques or any Traveller cheques for that matter for U.S. cash, peso’s only. The exchange rate is fair and convenient - you can get a slightly better rate at money exchanges (Cambio) in town or even at a bank.
If you need peso’s change in small denominations, you will not find places that will change back.
Please be careful when and where you use your credit card. There have been incidents reported of double billings and other charges being billed to credit cards used by tourists while in the DR. Credit card fraud is rampant all over the world so be careful.