Havana is a big city...
This city is large, with plenty to see. Allow a few days to hit a bunch of cigar shops, tour Habana Vieja, take a ride in a 1950s taxi and smoke plenty of cigars.
...but don't expect room at the inn
It's big but overwhelmed. Hotel rooms are full, so book quite early (months ahead) to ensure you will have a place to stay. The same goes for your flight.
Buy your cigars in the Casas del Habano
Don't buy cigars on the street. For about $8 or $9 you can get a phenomenal and legitimate Cuban in an official shop. Don't buy a fake. You're in Cuba—you want to smoke a real Cuban cigar.
American credit cards are now legal, but still essentially useless. Even other cards are seldom accepted. This is largely a cash economy. And the city is more expensive than you might expect, with $40 per person lunches and even more expensive dinners not out of the question. (read full text at Cigar Aficionado)