Hello! My husband and I have been to both of these amazing cities (Barcelona in 2017 and Lisbon in 2022), and you're in for a real treat! We're mostly foodies, so that will probably be the center of my recommendations, and we also don't know much about the nightlife (sorry!), as we're old and grumpy. But I'll do my best to offer some insights. There is so much to tell, I'm struggling to organize it all in my head, so I think I might just list things as they pop up, and please feel free to ask any questions!

Barcelona

• Tapas, tapas, tapas. I know this is a given, but exploring the small-plates scene is awesome. Every place has slightly different takes. My favorites: boquerones (not to be confused with anchoas (what Americans typically think of when they think anchovy)) - fresh anchovies pickled in vinegar, so they were not salty and cured, but plump and zesty; pan con tomate - fresh bread rubbed with olive oil, garlic, and tomato; croquettas - particularly ones with Iberian ham; gambas al ajillo shrimp cooked with garlic; olives - especially castelvetrano, if you like green olives
• Gaudi. If you like architecture at all - or even if you don't - it's worth seeing one or two places designed by Gaudi. Of course, the Sagrada Familia is a must-see (buy tickets in advance). If you only do one thing in Barcelona, see this. It's awe-inspiring and no words of mine can do it justice. We also recommend Park Güell as another fantastic Gaudi site (we didn't buy tickets in advance, but it was November, so we basically walked right in; in June, you probably would want tickets for this).
• Picasso Museum. We didn't want to wait in line, so bought tickets online the day of for a time about 2 hours ahead, and then just wandered and ate empanadas. If you love Picasso, this is worth seeing; considering we're not really fine art people, it was fine but not something I would do again.
• Drink the fresh orange juice! It's incredible. And also, don't be so tempted by the delicious sangria and beer that you forget to drink water. Yes, alcohol is cheaper than water; it doesn't mean your body can survive on it. This comes from personal experience...
• The Gothic Quarter is a very awesome place to explore and easy to reach by Metro. This is where we stayed, and we were able to walk to just about everywhere, but the Metro is quite accessible and convenient.
• If you like fish, the fried sardines from little hole-in-the-wall shops were very delicious! Worth a try!
• We didn't seek out any nightlife in Barcelona, but many places were open quite late, as dinner typically starts around 10pm. We would go out at 9 and be the early birds. Going to bed at midnight was late for us, but there were still many people about and things to do that late, so I'm sure if you're near a tourist center, there will be bars/clubs aplenty. Sorry I don't have any better recommendations for this, though!

Lisbon

• Bairro Alto is the neighborhood for nightlife. We stayed one neighborhood over, in Baixa, and there were still late-night things to do and we could hear people partying outside the flat until about 4am on weekends, even in this "quieter" neighborhood. I bet if you were in Bairro Alto proper, it would be even more lively!
• Cervejaria Ramiro is the restaurant I recommend if you like seafood. That's all they serve, it's so fresh they literally let you meet the crab before they cook it, and the prices are so affordable for the amazing quality. A dinner for two with multiple types of shrimp, a whole lobster, a whole crab, and a bottle of wine was about $150. They request a $50 pre-payment upon reservation, so we only paid $100 at the table. Very worth it, and probably the only reason we'd go back to Lisbon.
• Day trips! We were able to visit most of downtown Lisbon in a couple of days and had lots of time to kill (scheduled ourselves 10 days in this region), so I recommend looking for day trips outside the city if you'll be there for more than 4 days. Sintra was gorgeous, Cascais is a beautiful and touristy beach town, Sesimbra is a little town south of Lisbon known for its fish, Evora is known for its architecture and wine, and Coimbra is a little walled city that feels stuck in time. All of these are within a couple of hours and worth seeing, depending on your interests.
• Speaking of which, if you have time in your itinerary, I definitely recommend Porto. We both liked this city better than Lisbon! It's picturesque, has amazing food (sandwiches, such as the Prego and Presunto; or the popular Portuguese dish called Picapau), gorgeous architecture and so many azulejos (blue tile murals in and on buildings), and is excellent for learning about and drinking port wine. It's about 3 hours north of Lisbon by direct train and is just really lovely!
• Trains in Portugal are trickier than in other countries, from what I've seen. You will have an assigned seat, and they will walk through the train and make sure you're in it. But they're very helpful if you're lost (like we were). Train cars have their number on the outside, so first make sure you're in the right car. You won't know which way the train pulls in, so watch as it arrives to see whether your car is in the front or back. And then your seat number will likely be above the seat, on the luggage rack. Our motto for the Portugal trip became, "Welcome to Portugal! Good luck!" due to it being a little less obvious for non-native speakers than other European countries.

Etiquette
I can't really speak much to this, as I didn't notice any huge culture shocks in either place, aside from the regular "we're not in Kansas anymore" feeling. I'm an American from the West Coast, for reference. The people in both cities were extremely friendly and kind, and it was easy to ask for help when navigating the newness. That being said, as is always best practice, it goes a long way to learn simple, polite phrases in the native language. Practice these at home, so you feel less awkward using them. Folks really appreciate the effort! Also, both of these countries are heavily Catholic, so it's always a good idea to bring more modest clothing when visiting religious sites; I carried a pashmina that I doubled as a scarf and shoulder wrap for entering churches. The weather (especially in Lisbon) will probably be humid and hot, so it's important to remember to bring something to cover shoulders, even on sunny days!
My husband's advice is this - remember you're a tourist. He struggled with feeling anxious and out-of-place, and that hampered his time a little bit, until he remembered that it's going to be obvious you're a tourist before you even open your mouth. So lean into it! These cities are very tourist-friendly and used to it, so nobody will judge you about it. Just also beware that this makes you an easy mark for scams and pick-pocketing! So it's worth learning about ways to keep yourself safe from that, especially if you're staying out late at night. They're very safe cities, all-in-all, but you might still get robbed if you're obviously drunk or dumb.
I hope you have a great trip and enjoy yourselves!! Spain and Portugal are both wonderful countries, full of warm and friendly people who want to show you the best of their cultures. Take lots of pictures and keep a journal!