In Cuba, you can only spend Cuban pesos. Cuba has two currencies, a local currency and a tourist currency (CUC). The CUC has the same value as a US$. You cannot use a U.S. credit card in Cuba.

Best advise is to:
1) Allow for about $75 a day in spending money.
2) Change all your US$ to Canadian$ before you go. Many banks will do this for you with no exchange fee. Cubans do not like to exchange Mexican pesos, you will get a bad exchange rate. You can exchange US$ in Cuba, but you pay a 10% penalty up front and then there are exchange fees.
3) Exchange Canadian$ for Cuban pesos in your hotel or a bank for the best rates.
4) If possible get a credit card issued in Canada or Europe. It cannot be a credit card from a foreign bank, like HSBC, that was issued in the U.S.