It's usually always more expensive to drive only one-way with a rental vehicle remember, especially between countries. Train and subways are the way to go. A car in Portugal for a day or two, just to get out of Lisbon, and see a bit of the country might be a good idea. If you do decide to rent a car (in spite of the advice on here), if you are interested in getting anywhere quickly, stick to toll highways, the other routes might be prettier, but they would be endless. Although, is this a vacation or are you working?