Havana  is so large and the sights to be seen so many, that one week is the bare minimum needed. Metropolitan Havana sprawls over 740 square kilometers (286 square miles) and incorporates 15 municipios (municipalities). Havana is a collection of neighborhoods, each with its own distinct character.
If you have only one or two days in Havana, hop on an organized city tour . This will provide an overview of the major sites. Concentrate the balance of your time around Parque Central , Plaza de la Catedral , and Plaza de Armas .
Your checklist of must-sees should include the Capitolio Nacional , Gran Teatro , Fábrica de Tabaco Partagás , Museo de la Revolución , Museo Nacional de Bellas Artes , the Catedral de la Habana , and the Museo de la Ciudad de la Habana (in the Palacio de los Capitanes Generales ).
Habana Vieja  (Old Havana), the original colonial city within the 17th-century city walls (now demolished), will require at least three days to fully explore. You can base yourself in one of the charming historic hotel conversions close to the main sights of interest. Be sure to journey across the harbor to visit Parque Histórico Militar Morro-Cabaña , featuring two restored castles attended by soldiers in period costume.
Centro Habana  has many casas particulares but few sites of interest, and its rubble-strewn, dimly lit streets aren’t the safest. Skip Centro for Vedado , the modern heart of the city that evolved in the early 20th century, with many ornate mansions in Beaux-Arts and art nouveau style. Its leafy streets make for great walking. The Hotel Nacional , Universidad de la Habana , Cementerio Colón , and Plaza de la Revolución  are among the prime sights not to miss.
If you’re interested in Beaux-Arts, art deco, or even 1950s moderne architecture, then once-glamorous Miramar  and Cubanacán , west of Vedado, are worth exploring. Miramar also has excellent restaurants, deluxe hotels, and some of my favorite nightspots.
Most other sections of Havana  are run-down residential districts of little interest to tourists. A few exceptions lie on the east side of Havana harbor. Regla  and neighboring Guanabacoa  are together a center of santería and Afro-Cuban music.
Despite Havana’s great size, most sights of interest are highly concentrated, and most exploring is best done on foot.
All touristed areas are patrolled by police, but don’t let your guard down for a second.