It really depends what type of holiday you want, how long you are going for, whether you want to stay in one area, and so on
I like Sesimbra for a shortish family break; lovely beaches, nice restaurants, some interesting walking nearby, easy access to Lisbon and Sintra for days out. It’s a tourist destination for many Portuguese, but few other nationalities.
If beach holidays aren’t your thing (they aren’t mine, really), it’s worth considering the Upper Alentejo region, interesting and lovely mediaeval towns like Castelo de Vide, Alter do Chão, tiny Monsaraz, the fortress at Marvao and the marble towns such as Estremos. You’d need a car for this area though.