Catalonia has quite the long coastline, stretching almost 400 km, the Costa Brava is the most famous one, but you also have the equally interesting Costa Dorada (https://www.spain.info/en/region/costa-dorada/), which is located south of Barcelona. Some of my favourite points of interests:
Garraf - laid back beach community just south of Barcelona. Popular with locals and with good seafood restaurants. There is also here a beach house designed by none other than Antoni Gaudí!
Poblet Royal Monastery - a UNESCO World Heritage Site and still active 12th-century Cistercian monastery. It was a fortified medieval royal residence and contains the unique hanging tombs of the old Kings of Aragon.
Tarragona - ancient seaside town with very good Roman ruins, including an amphitheater, intact aqueduct, and Praetorium Tower, was once the capital of Roman Hispania (modern-day Spain). Tarragona's medieval old town is a delight to explore too.
Les Ferreres Aqueduct - Roman Aqueduct you can walk on top of, a few km from Tarragona.
Sitges - the quintessential posh beach town, 30 min south from Barcelona. Has multiple small sandy beaches. Known for its gay scene, Sitges also has nice historic centre lined with boutique restaurants and shops.
Castillo de Miravet - A large riverside stronghold built by the Knights Templar with commanding views of the countryside. The adjacent medieval village spills down the cliff towards the rivers. A great place to bicycle, kayak, or take a leisurely boat down the Ebro river.
PS: In Spain we have a little saying - "en Abril aguas mil" which means "in April a thousand waters", which means prepare for lots of rain in April, it will certainly not be beach weather. Also depending on your dates you may be coinciding with la Dia de Sant Jordi on 23 of April, an important day for the Catalonia, a bit like a Catalan Valentine's day, but with books.