Things I wished I had known before going to this country for the first time, hope it helps you!
Even if you are a seasoned traveller in Spain, Portugal will feel very different.
Nightmare queues are real, even in late September. Before I went, I read that I should go to each major tourist attraction (Jeronimos Monastery, Pena Palace, etc.) either (A) at the start of the day or (B) at its end. Turns out (B) is a much better strategy. These days, (A) means you will be waiting regardless of your first time slot as they give priority to tour groups over independent travellers. Touts and others will cut in front of you last minute anyway, so showing up early doesn't help. Also, staff may re-signpost lines last minute, so you will lose your place that you spent an hour queuing up for. Just go 1 hour before closing, by that time nobody cares about queues and you will have the place to yourself. Oftentimes entries to external areas (e.g. Pena Palace Arches Courtyard) and gardens may not require queuing. Queues may ONLY be for going inside buildings!
Pasteis de nata (pastel de nata is the singular form) tastes pretty much the same at any of the specialty shops. As a Portuguese told me with a straight face, "practicamente o mesmo". If there is a huge line at the one everyone heard of, Google the other handful of artisan makers and try those. It is better to taste a warm, freshly-made one. Portuguese sweet pastries and cakes focus on egg yolks and taste very similar; shapes and styles are different. More varied, interesting, and ignored by tourists are the convent sweets, for e.g. Doces Conventuais de Arouca in Porto, check it out online.
Uber and Bolt will save your life. Even for longer distances (e.g. Cascais to Lisbon Airport, Lisbon to Sintra), it's worth it. My Uber driver said he even once drove someone from Lisbon to Madrid (5.5 hours) when they missed their flight (why? See below re: Lisbon Airport). However, once I nearly missed a train in Porto on a Sunday because I got dozens of Bolt cancellations in a row (the app went crazy accepting/deleting nonstop). I eventually had to order a taxi and ran to the train with minutes to spare. I was trying to get to the train station from the historical center. The taxi driver said sometimes the police do not allow private cars into historical areas. Bolt still accepted my ride but then immediatelly cancelled without explanation, repeatedly.
Whether you are in a casual cafe or a fine dining restaurant, don't be surprised if waiter service is not what you expect. If you sit down and for eons nobody brings you a menu, put on a BIG SMILE and go up to talk to a waiter in clear English (unless you can speak Portuguese). In fine establishments they also speak French and Spanish. Remember to say obrigado (if you're a man) and obrigada (if you're a woman), and por favor (please), it really helps to be excessively polite/smile even in bad situations. If nothing is offered to you, communicate - ask for it nicely. If you keep quiet nobody will attend to you. My heart broke when a nice couple sat down, was stunned that they were ignored for ages, and got up again and left shamefacedly. Don't take it personally - I learned that things are really slow here. Bring a book.
Salad places like the Honest Greens chain (found in big cities) will save your life. That was the best advice I was given, pre-trip, because my friends knew I would get tired of meat and seafood.
Lisbon Airport - as of Sept 2025, the Arrivals lines at passport control (non-Schengen) are still horrendous (2-3 hours, if you are travelling with seniors or children, please note). Departures lines ok. Or, fly in via Porto Airport.
If you see seniors surprised by hills, stairs, struggling with luggage, fending off touts, please help them if they let you. At Lisbon Airport, you may have to walk a long way up/down stairs + shuttle bus to your plane. Wishing you all happy and smooth travels!