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When you visit Lisbon, it is important to see the old Alfama area, try a pastel de nata from Pastéis de Belém and ride on a tram along the quaint roads.
In Porto, you should go to the Ribeira district because of its bright houses and views of the river; also make sure to taste port wine in the warehouses over Douro River.
Regarding Sintra, the Pena Palace takes your breath away, and if you walk up to the Castle of the Moors you are rewarded with amazing views.
Concerning the ways of travel, public transport in Portugal is quite reliable, especially in urban areas.
Trains are good for moving from one city to another, while buses allow you to reach little towns or places less visited.
If you want adventure, hiring a car lets you go freely to distant locations, though it is not required if using public transportation suits your preference.
I hope this is useful to you.
Have a good journey and embrace all the beautiful things that Portugal presents for you.