If your accommodation is in the center of the city, walking is a great alternative. Many of the attractions of the city, such as the Castelo and the Alfama and Bairro Alto districts, are within easy walking distance of the Baixa.
If you become lost or cannot find the location you are looking for, try to locate the nearest Carris bus or tram stop. Most of these stops (not all) have a very good map of the city with your current location clearly marked on the map. All the prominent tourist sites in Lisbon are also shown along with an index at the bottom of the map. A quick consultation with one of these Carris maps should point you back in the right direction.
You may also use the funiculars and elevadores (Santa Justa's). Day passes for public transportation are also valid for those.