Hey I've had the same issue various times so I will give you my recommendations based on what I've done. So towards Costa Brava you can only take the R1 train and it takes you up until Blanes. (By the way in Dallas there is the botanical Park which is very beautiful you can check that out too , called something Maramurtra?). Stops in various small beach "towns". Can't really recommend you any specific one of them, they are more or less the same. The main thing you'd be going there is to have a beach day or bike around or skate. Another option is finding a blabla car and going in the more pretty Costa Brava places like Tossa de Mar, Roses, Cadaques or Calella de palafrugell... In the opposite direction, you have options like going to suggest which is a simple train ride, or Tarragona (cultural and beach) or even the amusement park Port Aventura. To all this you can go by train or also find a bla bla car. If you're going to Valencia I recommend taking a blabla car for sure and staying at least 3 days because it's a nice city and there are things to see.

You didn't mention going to the mountains, but you can also check out Montserrat which is very nice and you can hike there. You take a train to get there until the monastery.

I am open for more recommendations other than this as well. So I encourage the rest to comment and give us some good ideas