If you are into visiting beautiful monuments and enjoy remarkable views, then Lisbon, Sintra, and Porto are the top three places, and all of them are well worth a visit. But don't overlook Viana do Castelo, Braga, Guimarães, Coimbra, Tomar, Aveiro, Amarante, Bragança,Chaves, Lamego, Viseu, Vila Real, Lagos, Silves, Évora, Angra as they also have wonderful monuments and places of interest. Although in Portugal most of the monuments in existence are somewhat neglected from a tourist point of view, one can easily find very old romanic churches, some with almost 900 years and still quite unchanged from that time. Old monasteries like Alcobaça or luxurious palaces like Mafra one of the biggest palaces in Europe or Queluz are not to be missed either. In most of these places you will have no queues.
The most popular beaches are in the Algarve, which has stunning coastlines and gobs of natural beauty. The water along the southern coast tends to be warmer and calmer than the water along the west coast, which is definitely Atlantic and doesn't benefit of the Gulf Stream. For surfing, or just playing in the surf there are great beaches all along the west coast, near Lisbon and Peniche. Don't forget also some of the almost deserted beaches along the Costa Vicentina, in Alentejo.
For nightlife Lisbon, Porto and Albufeira, Algarve are the best choices as you have major places of entertainment.
If you want to spend your holidays in the countryside, you might want to visit Viana do Castelo, Chaves, Miranda do Douro, Douro Valley, Lamego, Tomar, Leiria, Castelo Branco, Guarda, Portalegre, Évora, Elvas or even Viseu.
And even if you wish to observe wild life in its natural state, Madeira and Azores Islands are places to remember, not forgetting of course the Natural Reserve of Peneda-Gerês, the Douro Valley and Serra da Estrela.