Bring sunscreen, shampoo and mosquitoes from your country. Although it is possible to get them in Cuba, these products can be more expensive on the island, especially the protective cream. In the CADECA of the first floor of the airport you can get national pesos.
 
Arm yourself with patience during your stay and get ready to wait long lines. This country is unique and many things work differently. Try not to complain about things that don't work, and less, in front of Cubans.
 
Beware of tap water. In theory it is drinkable, but I do not recommend it. Many travelers who take it end up with diarrhea.
 
In Cuba it is not easy to find free city maps. I recommend you download them to have super-detailed offline maps. It is important that you install the app before going to Cuba because you may have problems accessing the app store from Cuba.
 
Bargain for almost anything you want to buy (because they usually want to charge you a lot more), but remember that they have very low salaries. Do not press too much. Confirm that you have all the necessary documentation before leaving: passport in order, round trip ticket, tourist card and medical insurance.