If you're American, you may only spend US dollars. Travelers checks and credit cards drawn on US banks are not honored. The US dollar is the currency of choice, and chances are, you'll never spend a peso on your entire trip. No guarantees, it's just what we're hearing. Buses are generally best avoided, and they're usually very crowded and behind schedule.
Trains are also an affordable option.
Trains leave regularly from Havana, and amongst other cities, go to Matanzas, Santa Clara, Guayos, Ciego de Avila, Camaguey, Las Tunas, Cacocum and Santiago de Cuba, Bayamo, Manzanillo, Guantanamo, Baracoa, Holguin, Antilla, Cienfuegos and Pinar del Rio. There are also many smaller airports. Departing from Havana, regularly scheduled are flights to Santiago, Holguin, Camaguey, Bayamo, Manzanillo, Guantanamo and Las Tunas. Also, there are charter airlines operated by tour operators. In fact, violent crime is almost unheard of in Cuba, making Cuba by far the safest of the Caribbean islands.