Carry cash and change with you

Cuba is almost entirely a cash economy. Almost no restaurants, taxis or tour operators accept credit or debit cards. So, it’s important to take out and hold a lot of cash.
It can be a pain getting CUC all the time, small towns won’t have ATMs and the banks / CADECAs have relaxed opening hours. So, get a lot out early. It’s a safe country and you are unlikely to be separated illegally from your CUC as long as you take sensible precautions.

Havana international airport has 5 cash machines as you exit customs and another one on the floor above. There is also a couple of banks and a CADECA, for which you will probably be queueing for a while.

Finally, because of the dual currency system when you hand over your CUC make sure you get CUC back. Those CUP really are not worth much.