Due to the MLC and the declining value of the CUP, locals are very interested in foreign currencies. It is therefore very easy to pay in USD or EUR.
It is best to bring small bills. Where 5 EUR bills are too large, you can pay with smaller USD bills (never coins in foreign currency). You can pay in foreign currency almost everywhere and usually get a better price. However, change is usually only given in CUP. The exchange rate is a matter of negotiation.