Last month my wife and I enjoyed several days in Lisbon. Renting a car would be a hindrance, as Portugal has an excellent public transportation system and taxis are very available and inexpensive. Suggest you take a walk in the Baixa area, take tram #28 through the Alvama neighborhood. Enjoy meals in the excellent moderately-priced outdoor restaurants. Take a tram, bus, or taxi to the nearby Belem suburb to tour the Geronimo Monastery and visit the Monument to the Discoverie and the Belem Tower. Take the train from the Rossio Station to Sintra (trains every twenty minutes for the 40-minute ride) to visit the Pena Palace and the National Palace. If you have time in Lisbon also visit the Gulbenkian Museum, a world-class international museum. For details and photos see the "Portugal" section of Our Travel Journals: http://www.evcal.org .
Pro tip
