Something pretty simple and free to do in town is to walk around Alfama to enjoy the whole scenery Lisbon can offer.
You have to know that Alfama is the oldest part of Lisbon and the second oldest area in Europe, which dates back to the Moors of North Africa that invade Portugal.
Actually, the Arabs created first Alfama in the same type of architecture that you could find in Morocco.
For that reason, you should explore the alleys and narrow streets that form the district of Alfama.