Lisbon Tips:

Dude. This is a weird thing, but also might save your life. The sidewalks are made of tiles. These tiles are INCREDIBLY slippery. Rain, sun, it doesn't matter. My first day in Lisbon I tallied the number of times I almost fell on my face from slipping all over the place and it was 26. Good luck!
- Lisbon is also really hilly. So prep for sliding down a hill, whether you're actually trying to go up or down.

Sit by the river (or really anywhere outside) and relax with a drink. Preferably sangria. Lisbon has a great culture of outdoor drinking since their weather is pretty much always perfect.
Mercado de Fusao: Outdoor park/food trucks/music/bars, definitely a cool spot

Just poke your head into a sardine store. There's a bunch. It's crazy to see
Praca do Comercio: The center of the city, right on the river, surrounded by gorgeous buildings
Sao Roque Church: Gorgeous interior + the park right behind the church provides great views of the city

Barrio Alto neighborhood: Lots of bars and restaurants, but it is definitely more popular at night than during the day
Time Out Market: Tons of food options and generally a pretty cool spot
Belem Tower, Jeronimos Monastery, Monument to Discoveries: All three of these attractions are near each other outside of the central city. All three are worth a visit, but if you go to the Monument to Discoveries don't pay to take the elevator (unless you need to) it's not worth it and you can easily get sucked into it because it does not seem like you have a choice

Sintra: Check out Quinta da Regaleira, it's essentially a small castle and a pretty park, then wander around the city center
Cabo da Roca: The end of the world! That's what Europeans thought until they discovered the Americas. Personally I combined this with my trip to Sintra and it worked out very well.
Cascais: A gorgeous beach town a short train ride from Lisbon. Check out Boca do Inferno and then lounge on the beach!

Porto Tips:

This may be obvious but… drink port. Port wine is definitely an acquired taste, with the heavy alcohol taste really shocking you at first, but after a couple samples it'll taste smooth!
Make sure you explore the areas not centered around the river too. It's easy to just marvel at the river and the surrounding landscape, but there is lot to see in the rest of the city as well! However, Porto is a little sketchier than other European cities, so make sure you're careful, especially at night!
If you're a vegetarian (like me) : good luck. I've never struggled to eat somewhere as much as I did in Porto. If you see a menu that has food you can eat, jump on it!

Ramos Pinto: My top recommendation for port wineries (and trust me, I tried a lot)

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Lisbon, Portugal