15 days in Spain in July

Have you read the Spain guidebook? https://store.ricksteves.com/shop/p/spain-guidebook

You could also look at the tour itineraries and get ideas on how to structure your trip https://www.ricksteves.com/tours/spain-portugal/spain

OK, on day of arrival either fly or train directly to Granada. You are already in long travel mode so know the whole trip out at once. Flights within Spain can be inexpensive and since you are already at the airport save some time by not needing to transfer to the train station. BTW, if you must, the train is a great mode of easy travel.
https://www.seat61.com
Spend three days in Granada
Then fly to Barcelona, cheap flights again, and spend 6 days. Huge menu of opportunities and a great place to swim like a locale on the beaches. Day trip opportunities abound.
Then head to Madrid via AVE/train for remainder of journey.
Enjoy
STAY HYDRATED!

I would suggest a day trip to Toledo from Madrid
Also , spend some time in Seville Granada, Cordoba. The paradores are wonderful places to stay and often times in historic towns.

That time of year it may be difficult to find a beach anywhere near Granada or Barcelona that is mostly locals. An area of Spain where fewer tourists visit, so beaches might have more locals, is southeastern Spain between Alicante and Almería. My guess is you won’t be conversing with many on the beach, so I’m perplexed as the why the locals vs foreigners, or how you’d know by looking at them which is which. There there many nice beaches north of Barcelona and also between Barcelona and Valencia.

To me Granada needs no more than 2 full days/3 nights to see while you be shortchanging Barcelona with anything less than 5-6 nights. What you want to see in Madrid would determine how many days or nights you’d need.

Thank you all. I have read Rick’s guide books. The reason for a less touristy beach experience is crowds although that might not be avoidable. Definitely spending a majority of days in Barcelona.

Following on the above posting, your ambitions for Madrid might dictate if you even have time for a stay in a beach town.

You can also do what the Barcelonaeses do, and take the commuter rail for a day trip to a beach town. Or the subway to the beach in Barcelona itself.

Europe has a much higher population density than the US. Northern Europeans don't get much sunlight in the winter, so a lot of them head to southern beaches in the summer. Those two facts make for extreme crowding (by US standards) on the nicer, more accessible beaches. It is best to moderate your expectations.

Totally agree. Too often visitors ask for "secret beaches" or "quiet coves"... but in summertime, there aren't any in reality.