The Jerónimos Monastery or Hieronymites Monastery is a former monastery in the Belem area of Lisbon. Today it stands as a national monument, museum and UNESCO world heritage site.  It like the tower of Belem is one of the main tourist draws to the city and is most certainly worth the trek if you are staying closer to the Alfama or Baixa areas of the city. Board Tram 15 at the Praça do Comércio and exit at the Belem stop.

Tickets for the monastery, Belem Tower, architectural museum and art museum can be bought at the main counter at the monastery in either single ticket or bundled in various ways depending on what you want to see.
 

The monastery itself is a quick see, it consists of a gorgeous court yard, a small exhibit downstairs, a small but comprehensive over view of Lisbon, Portuguese and world history on the second floor and then of course the upstairs viewing of the gothic Church of Santa Maria.

To walk around the main level of the church, you must go back downstairs, exit the monastery where you came in, and then re-enter the lower area of the church at the main door which is across the walk way from where you would have bought your tickets.
 
Location Hours and More Information:

Praça do Império, Belem
10AM-5PM (Oct.-April), 10AM-6PM (May-Sept.) (Closed Mondays)
To get there from Baixa (Praça do Comércio) take Tram 15 and exit at the Belem stop.
Museum Site
Other things in the area include; Maritime Museum, Archeology Museum, Tropical Gardens, Belem Palace, Coach Museum, Electricity Muse am, Princess Fountain and more.