We are recently back from Portugal, but of these, only did Lisbon. We enjoyed the Castle on our first night, arriving 2 hours before sunset. We did book the tickets ahead as many suggested.
For getting a taste of the city (all you have time for), we enjoyed strolling through Alfama, taking the streetcar to the area near Praca do Comercio, strolling to the TimeOut Market, and talking a bolt to Belem. We skipped the main sites there and did the QUAKE museum--cool if you like interactives and history.
We also spent time wandering the Baxia and Chiado neighborhoods as well as doing the Santa Justa Elevator and then walking down through the Chiado side. We did not have time for Sintra but still felt we got a taste of Lisbon and a desire to return.