Belem - located in the southwest corner of Lisbon - is a perfect place to spend a leisurely, sunny afternoon. Some of the most iconic monuments of Lisbon are found here, namely The Jeronimo Monastery, The Monumento do Descubrimento, and the Torre de Belem.
My advice is to take your time strolling through the area. Stop to people watch in the park in front of the Jeronimo Monastery. Soak up some sun at the edge of the river next to the Monumento de Descubrimento. Have a gelato before walking over to Belem Park and stretching out in the shade of a tree for a while.