will be in Barcelona in mid-October. Advice plz on best beaches there or within an hour or so. Thanks.

Google the Costa Brava, it's beautiful.

Casteldeffels and Sitges are accessible by public transportation.

you may walk on the beach in Barcelona in mid Oct, just bring an umbrella.
I visited Barcelona the first week of Oct twice... was sunny ( but cool), rained several times , hardly beach weather (60s)

Barcelona has a wide sandy beach right in town (Barceloneta and further northwards up to the Porto Olimpico and even further northwards). The beach is not super-attractive but not bad either. For a stroll or swimming certainly okay. Be a little careful with valuables.

As said, just south of Barcelona, there are the wide, sandy beaches of Casteldeffels and Sitges which are a little more attractive.

The beautiful, cliff-bordered coves of Costa Brava are at least 1 1/2 hours north of Barcelona, beginning with Tossa de Mar and northwards.

Barceloneta is not nice but it will suffice on a hot day, better than Madrid anyway.

Barcelona is a great city and in mid OCTOBER ( when OP is going) a place that offers much more than Santander or San Sebastian.( so does Madrid)