I think we have much of the same interests :-) Here are some fun places I would recommend to check out between Barcelona and Valencia (north to south) close to the coast:
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. (car needed)
Villa Romana dels Munts - a 1st-century Roman villa overlooking the Mediterranean, with preserved mosaics, baths, and frescoes that once belonged to the Roman governor of Hispania. (car suggested)
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.
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. (car suggested)
Peñíscola - With its cobbled streets and whitewashed houses, the old town of Peñíscola sits on a big rock that juts out into the sea. The town is dominated by a large Knights Templar castle that looks like something out of the Crusades. Recently, Peñíscola was used as a filming location in the T.V. series Game of Thrones.
Sagunto Castle (Saguntum) - settlement sacked by Hannibal Barca that led to the Second Punic War. Now an eclectic mix of Roman, Moorish, and Christian fortifications. (car suggested)