Definitely bring cash. Although some places accept US bank credit cards, some don't. Also, sometimes a place that accepts CCs just isn't having its equipment working properly that day. As for exchanging money, Cuba penalizes travelers converting US dollars to the tune of 10%. That is in addition to the 3% exchange fee. Euros don't have that penalty so exchanging dollars to Euros here at your bank and then changing the Euros to CUCs in Cuba nets you more than if you exchange dollars in Cuba.