Off the beaten path sites

Hi Sue... that's a very vague question I must say... unless you share what your likes and preferences are :))

Do you mean alternative day trips, or visitors' sites like Mesquita al Zahra just outside Cordoba (Moorish), or Italica near Sevilla (roman - think 'the Gladiator' movie), or Palacio de las duenas in Sevilla (the most recent Duchess of Alba was a real character and worth reading up on before visiting)? There are a lot of sites that aren't necessarily out of the way, but Rick doesn't make note of. I suggest your get an alternative guide book either as an ebook download (I am using Lonely Planet this way - they sometimes have inexpensive promos for discounts on their ebooks) or from your local library. Even a book slightly out of date will give you alternatives to Rick's considered highlights. Or lurk on TripAdvisor's 'Things to do' for each page. You don't have to take their review recommendations, but you can at least see a list of what people are seeing.
I am heading to Spain in October with RS as my paper guide book. I have been adding in Post Its, pencilled in marks of all sorts of sites within these cities that don't make his cut.

Look into Baeza and Ubeda north of Granada.

Out of the way spots? Do you mean where there are few tourists? Hard to find places like that in 4 cities that are top tourist destinations. The most interesting spots are the ones that everyone goes to.

Last time I was in Barcelona I went to the ethnological museum on Montjuic and the Hash Museum in the Barri Gotic. I thought both were interesting and enjoyable and there was hardly anyone in either, but I was there in off-season - early March. But that was toward the end of a week-long second visit. I wouldn't recommend them instead of more well-known sights.

The Duchess of Alba's home that Maria suggested isn't in all the guidebooks yet because it's only been open for a little over a year. It's definitely worth visiting if you have the time.

The Sant Pau Recinte modernista site in Barcelona is wonderful and as of last August was very lightly visited. I do not expect that situation to continue for very long.

Barcelona has a lot of quality museums, most of which are not heavily trafficked; it's just a matter of doing a bit of research to see which ones seem to align with your interests. The Picasso is a zoo. MNAC and Miro were busy but manageable on the days I saw them.

Jaen is good to see and different. Look and the Parador Jaen next to a Santa Catalina Castle.
And Frigiliana a white village is lovely.

Thanks everyone. I didn't specify interests because you never know what tips come up without direction.

I found a fascinating website atlasobscura.com that shows unusual sights for anywhere in the world. You can choose Atlas at the top, then the region/city, to see unusual options. Or choose Stories > Explore the Map (my preference, as it shows everything in map vs list form). Just briefly looking at Barcelona, I see the Giants of Santa Maria del Pi, Barcelona Erotic Museum, and the Barcelona Baby Drop. It also shows the usual sights to see, but the map is helpful to see what may be near other places you'll be visiting. We have found several off the beaten path things to see with this website.