This summer I will be travelling to Helsinki (and staying there for about a week). While both countries belong to the EU, and so I am not obliged to get a passport, I know that not having one can make your life difficult if you travel to some (EU) countries - for instance, the UK. Considering this, would you (as experienced travelers!) recommend me getting a passport before I leave?
Because you are an EU national you have the right of free movement inside EU.
The national ID card is enough, the portuguese one is called Cartão de Cidadão.