Been there twice via Mexico (CUN) - the only thing that gave me heebie-jeebies during the trip was yes, you have to bring a big wad of cash and more worrisome - you have no source of more if somethings happens.

First time there brought Canadian money. After the first few days realized my cash burn-rate was higher than expected and started to panic - you can't just go to the ATM or bank and get more cash, you are pretty much on your own. Had to adjust and be more conscious of the spending and it worked out fine.

Second time I didn't bother getting the Canadian money. You get dinged something significant when you exchange US Dollars for CUCs (10%?) but I found that getting the Canadian money in the US, then exchanging back the leftover double-dinged me for exchange fees, plus the hassle factor.

Exchanging US dollars is not a problem at all, no matter what you exchange, do as much as you can at the airport when you arrive, that was the easiest currency exchange of the entire trip, all the other locations were open bad hours, had long lines and the wait was hours in some cases, you don't want to do it any more than you need to.

Euro is super easy to exchange too. I think even some of the hotels can change it for you as well.

Loved Havana, frozen in time and the people are great, nowhere else in the world like it, you need to see it before Starbucks and the Gap move in.