An easy way to travel around Cuba is by bus. The state owned bus company Viazul has a wide network covering most destinations that are of interest for tourist. The buses are fairly modern with aircondition. And it is a very inexpensive way to travel. For example, a ticket from Havana to Varadero cost only US$ 10. And a ticket all the way from Havana to Santiago the Cuba cost US$ 51. (March 2016).
Our advice is to book tickets well in advance. You can make reservations without having to pay in advance. The buses quickly get full and they do not add extra buses. You cannot count on getting a ticket if you try to buy it on the same day as travel.
Here you find the Viazul home page where you can make reservations and pay the tickets online.