First go north (Braga, Guimarães and Geres Natural Park). Then do the boat trip in douro valley (come bank to Porto by train). Heading south I would say stops at Aveiro and Coimbra. In center Portugal. you have Alcobaca, Batalha, Nazare, Obidos and Tomar (here you'll find some transportation issues). In Lisbon's area you should visit Sintra, Cascais and Arrabida mountain range. To the south visit Evora.
You have much more but if you achieve this you're already seeing a lot. Enjoy! over 6 years ago