Most of the interesting areas of Portuguese cities (historic centers) are small so they can be visited on foot if you have the time and the inclination. There are many different means of transport in Lisbon, ranging from antique trams and elevators to modern metros. Lisbon is famous for its 7 hills that can be climbed using the century-old tram, the funicular, or the lift. There is also an extensive subway (Metro) and bus (Carris) network crossing the city.
Lisbon's public transport is not expensive (compared to transport in other capital cities in Europe). Nevertheless, if you travel often on a certain type of public transport it is smarter to either get a monthly pass or another sort of temporary pass (Lisboa Viva Card and Zapping).
Taxis are relatively cheap. The price starts at €3.90; if you share the costs it can be cheaper than traveling by bus or tram.
It is easy to travel from north to south using the Portuguese highways. However, remember that in Portugal highways are not toll-free, so you will have to pay a fee to use them.