Daytrip from Barcelona-with waterfront views
 

The Costa Brava just north of Barcelona has many waterfront towns.

I've heard of Costa Brava. But do you know of a particular town to recommend there? And how to get there.? Thanks!!

Try Tossa de Mar. It has a lovely old town, a castle and lots of choice for eating and drinking with sea views. You can catch a train from Barcelona to Tossa with lovely views from the window.

dkdrasta,
If I'm reading your other thread correctly, you'll have just June 2-3-4 to sightsee in Barcelona. Since your time there is short and there's a great deal to see, I wouldn't attempt to take a day away from Barcelona to go as far as the Costa Brava just for water views.

To reach Tossa de Mar on the Costa Brava, the commuter train ("rodalies") R1 only goes as far north as Blanes, then you must take a Pujol bus to Tossa.
There's no direct train from Barcelona to Tossa.
The direct Moventis Sarfa bus from Barcelona is a better option, departing from Estación del Norte. But the ride still takes 1 hr. 20 min.

An easier and much quicker way to see a different waterfront town is to go south to popular Sitges on the commuter train R2 that departs every 20 min. or so from the commuter rail station Barcelona-Passeig de Gràcia in the center city. The ride to Sitges takes between 38-43 minutes.
Lively Sitges has an expansive beach and beach promenade with plenty of places to drink or dine and some pretty Moderniste architecture.
But...it will get very busy on the weekend of June 3-4.

See the timetable here
https://rodalies.gencat.cat/en/horaris/index.html

And you can walk along the sea on Barcelona’s own beach, Barceloneta.
However, you have very little time to see Barcelona itself. Skip the idea of any day trips. We spent a week in Barcelona and never left the city

Same with us on our first trip to Barcelona !

On subsequent visits, we took day trips to Girona, Sitges, Tarragona and Colonia Güell.
The waterfront in Barcelona is long and worth a look.

If you decide to stay in Barcelona rather than take a day trip (and I completely agree that you have very little time for ANY day trip) and if you want a pleasant place right at the beach to have a drink or meal, consider Xiringuito Escribá.
It's run by the same family of the famed Escribá pastry shop. It sits on Barcelona's PlatjaBogatell (Bogatell beach)
This would be a very good place to enjoy a Saturday afternoon paella lunch on June 4, but you should book now online, as "weekend paella by the beach" is a very popular activity for Barcelona families and visitors alike.

https://restaurantsescriba.com/xiringuitoescriba/en/

Thank you. And which was your favorite day trip. If you only went to barcelona once for 4 days? Would you take a day trip? To have some variety and get out of the city? Which one and why??

For only 3 full touring days, I wouldn't take a day trip simply because there's so much of merit to see in Barcelona. If you're at all interested in all the multiple Moderniste sites (Art Nouveau) plus Picasso and Miró museums plus all the rest (Gothic Quarter and beaches), you'll be hard pressed to see them all in 3 full touring days. Just some thoughts.

As someone who likes the city and all it has to offer, I agree with Maribel .But if you still wish a change, she gave you a perfect
destination: Sitges. Easy to reach from the centre of Barcelona, nice long beach, number of seaside restaurants..etc. Half a day would do it.
As M. mentioned…many places ,restaurants and trains get crowed on weekends.

Sitges train has been the answer for many years now, Mrs Bilbo took it with her parents over 50 years ago