When travelling in Portugal, the drink of choice is wine. Red wine is the favourite among the locals, but white wine is also popular. Also Portugal along with Spain have a variation of the white wine that is actually green Vinho Verde. Its a very crisp wine served cold and goes best with many of the fish dishes. Drinking wine during a meal is very common in Portugal, and also after the meal is finished people will tend to drink and talk while letting their food digest. Don't let yourself be bullied into drinking if you're driving, though!
Port wine may be an aperitif or dessert. Alentejo wine may not be worldwide known as Porto, but is quite as good. Portugal as also other defined wine regions regiões vinhateiras which make also some of the very best of wines like Madeira, Sado or Douro.
Folks might find it a bit difficult to refrain from drinking, even if there are very good reasons to do so such as the above mentioned driving. Nowadays the "I have to drive" excuse works ok. The easiest way is to explain that one can't for health reasons. The Portuguese aren't as easily insulted as others when it comes to refusing the obvious hospitality of a drink, but a lie such as "I'm allergic" might make clear a situation where one would have to otherwise repeatedly explain a preference in some regions of Portugal; but it won't work in other regions where obviously made-up excuses will tag you as unreliable "I don't want to, thanks" might then work. Drinking is considered almost socially intimate.
Be careful of 1920 and Agua Ardente burning water, both pack a mighty punch.
Portugal is well known as the home of Port wines.
The legal drinking age in Portugal is, since 2015, 18 years old.
Porto is famous for the eponymous port wine, a fortified wine 20% made by adding brandy to the wine before fermentation is complete. According to EU laws, port wine can only be named as such if the grapes are grown in the Douro valley, and the wine is brewed in Porto. The end product is strong, sweet, complex in taste and if properly stored will last 40 years or more.
There are many, many grades of port, but the basic varieties are:
Vintage, the real deal, kept in the bottle for 5-15 years, can be very expensive for good years. It is, nevertheless, worth it.
Late-Bottled Vintage LBV, simulated vintage kept in barrel longer, ready to drink. Nice if you are on a budget.
Tawny, aged for 10-40 years before bottling, which distinguishes itself by a more brownish red color and a slightly smoother bouquet and flavour. As with any wine, the older it gets, the more rounded and refined it will be.
Ruby, the youngest and cheapest, with a deep red "ruby" color.
White port is a not-so-well-known variety, and it is a shame. You will find a sweet and a dry varietal, the latter of which mixes well with tonic water and should be served chilled if drunk alone or with lots of ice with tonic, commonly used as an aperitif.
Another good choice is the ubiquitous vinho verde green wine, which is made mostly in the region to the north of Porto the Minho. It's a light, dry and refreshing wine approx. 9% -9.5% in volume, made from region specific grapes with relatively low sugar content. Mostly white, and sometimes slightly sparkling. Very nice, and very affordable.