How should a first-time traveller plan Portugal without overpacking it?
Start with Lisbon and Porto. With seven days, those two cities are enough. With ten days, add one place such as the Douro Valley, Coimbra or the Algarve. With two weeks, the Alentejo or a slower coastal journey becomes easier. Do not add every region simply because the train or motorway makes it possible.
Full answer
Portugal is small enough to cross in a day, but a good first trip should not feel like one long transfer between famous names.
Lisbon needs at least three or four nights. The city is hilly, its neighbourhoods feel different and many of the best moments are not major sights: a morning coffee at the counter, tiled streets after rain, the ferry across the Tagus or dinner somewhere you found while walking downhill.
Porto deserves two or three nights. The centre is compact, but the city is more enjoyable when you can see it after the day visitors have left the Ribeira. Walk across the upper level of the Dom Luís I Bridge, return along the river, and keep time for the food and wine rather than treating Porto as one photograph from the opposite bank.
With those two cities, you already have a complete first trip.
What to add with more time
The Douro Valley is the easiest addition from Porto. A day trip shows the scenery, but one night is much better. When the tour buses leave, the river becomes quiet and the hills feel far removed from the city. A car gives more freedom, though trains and organised trips work for travellers who do not want to drive.
Coimbra fits naturally between Lisbon and Porto. Stay overnight if the university, old streets and student traditions appeal. Otherwise, it can become a luggage stop that takes time from both cities.
The Algarve suits travellers who want beaches, cliffs and several slower days. It is not one town. Lagos, Tavira, Faro and Sagres create different holidays, so choose the base before deciding how many nights to give it.
The Alentejo suits travellers who want whitewashed towns, cork country, wine and roads with little traffic. It works best with a car and at least three nights. Évora alone can be visited by train, but the wider region is the reason to add it.
A good trip by length
Seven days
Spend four nights in Lisbon and three in Porto. Add a day trip only if it matters more than another day in the city.
Ten days
Spend four nights in Lisbon, three in Porto and two or three in one other place. The Douro and Coimbra are the easiest additions. The Algarve can fit, but it uses more travel time.
Two weeks
Add the Algarve or Alentejo properly, or slow the trip down with longer stays and a few carefully chosen day trips.
What to leave out
Do not add Sintra, Cascais, Óbidos, Nazaré, Coimbra, the Douro and the Algarve to one short trip.
Sintra can use most of a day and becomes crowded. The Douro is better when it is not squeezed between a late arrival and an early departure. The Algarve deserves more than two nights if the beaches are one of the main reasons for visiting Portugal.
Choose the places you most want and let the others remain possible future trips.