I will arrive in Barcelona in May. The only trip on my bucket list is the Guggenheim Museum in Bilbao. Other than that I have no agenda. No desire to see famous places, things. Mostly would just like to travel and mingle with the natives. I am fluent in Spanish.
I am hesitant about booking lodging in advance because I may like where I am at and not want to move on. Is this realistic? Or am I dreaming?
Find a place on the Ramblas to watch tourists and mimes. Great way to do nothing.
Can’t help you with Bilbao, but I’ve been all over Portugal otherwise. Being fluent in Spanish likely won’t help you a whole lot. Portuguese is different enough to be difficult to understand, and most Portuguese mumble a lot. 
There are a lot of pousadas in Portugal that can be fairly reasonable or expensive, depending on where it is. Same for pensiones in Spain.
I’m not sure why you would avoid the sights in either country. Beautiful architecture with Moorish influence abounds, and the Manueline architecture in Portugal is interesting. The Portuguese aren’t minglers when it comes to strangers, so your attempts to be chummy will likely be rebuffed.
If you just want to wander around small Spanish towns, I’d suggest Andalucia.
If you get out towards Madrid, try to get to Toledo. Also, the Parador system of hotels is kind of designed to let you find a place while you’re wandering. They are all over. Some are historic (I stayed in thisone outside of Seville) and some are more like resorts (golf, etc.)
I never make reservations when I travel. I’ll call or make reservations on hostelworld (which also books guesthouses and small hotels) a day or so in advance.
Only once in my life have I ended up with no place to sleep- in Allahlabad India in 2003. There may be times when you have to pay more or stay in less than ideal places, but I find it worth the flexibility.
I’m not a big fan of art or art galleries generally speaking, but I really enjoyed the Salvador Dali musem which is near Barcelona. So if you’re going to be in the area, I’d recommend it.
Thank you for your post Chefguy.
It is not that I want to avoid the sites, I just don’t want to make them a priority.
Thanks for the input on Portugal. May not go there.
Andalucia looks great. Thank you.
Thank you. Toledo is now on my route to Bilbao.
Super-duper! I arrive in Barcelona. Thank you!
I would visit San Sebastian. We stayed in the old part of town. The food there is amazing. Every little bar had amazing pintxos (tapas). There was a lot of sea food and just a lot of bars. I had an amazing time. The city was pretty and the weather was much cooler than the rest of Spain.
Don’t get me wrong: Portugal has a lot to offer. Cheese, bread, wine and a pleasant climate are all good reasons to visit. You can go sit outside a Brasileira coffee shop in the Bairo Alto district of Lisbon and watch the world go by while chatting with the statue of the poet Fernando Pessoa, or go to the Alfama district, which is the old Moorish 'hood below Sao Jorge castle and wander the steep, narrow and winding cobblestone streets, then plunk yourself down in another coffee shop and have a 'bica while admiring the azulejo tiles that are typical all over the city.
Gracias! It looks incredible. Got it bookmarked. Well, I don’t have a book. Let me say, I have noted it.
Consider a couple of days in Girona. It is nearby Barcelona and is such a nice city. It is the city known for pro-cyclists to winter in, and Mr. Evil hisself (Lance) spent many winters there. There are a lot of nice hotels at reasonable prices and the same for restaurants. Maybe two days is enough.
You will love the Guggenheim. It really is a beautiful structure. Bilbao is a great city with a number of other architectural beauties. It is nice to travel west from Bilbao to many towns along the coast.