I went to Havana last week for a couple of days. Flew SNA-PHX-CUN using UA miles on US, then Mexicana from CUN to HVN and back HVN-CUN on Cubana. We stayed at the Hotel Nacional (5*)for $120 per night and booked the hotel, Mexicana and Cubana flights thru a Canadian Travel Agent called
www.wowcuba.com with a US credit card. It was very easy
I highly recommend seeing Cuba before things change and it becomes another Cancun.