United has round-trip flights from Dallas (DFW) to Havana (HAV) for $302-$351 with a connection. Flights March to May. No Friday departures. Extended weekends available. Origin/Destination reversible.
To enter Cuba as an American citizen, you must have a valid Tourist Visa, proof of a return flight back to America, a valid passport, proof of medial insurance, and the certification travel form (that outlines the reason why you are traveling to Cuba). You can apply for the Certification of Travel on the OFAC website, linked.