The coast south of Barcelona

Montserrat with its mountain-fringed monastery, the old town of Girona, and the coastal towns of Tossa del Mar and Lloret del Mar make great stops.

Do you mean you're both starting and ending in Barcelona, or are you traveling to Barcelona by car from somewhere to the south? It would help to know your starting point.

As a big lover of modernista architecture, I found Reus, Gaudi's birthplace, a very nice place to visit. It has no Gaudi-designed buildings, but it a good bit of modernista architecture by others, including two visitable sites, Casa Nevas and the Instituto Pere Mata. There's also the Gaudi Centre Reus. Reus is NW of Tarragona; I'm not a fan of Roman ruins, so I haven't seen Tarragona itself.

Sitges is an artsy beach resort with some museums.

If you're starting way farther south, you have Valencia. From there the very interesting Teruel would be a considerable detour.

Kenko's suggestions are good, but they're all north of Barcelona and you mention the south. So, let me share my thoughts about the south!
The coast south of Barcelona is not my favorite part of the country, but there are a few sights between there and Valencia. The following comes to mind:
- Sitges: lively seaside town
- Tarragona: walled city with Roman ruins
- Tortosa: riverside town with interesting medieval churches and a castle-turned-hotel
- Delta de l'Ebre: wetlands with birdlife
- Peñíscola: striking old town on a península surrounded by the sea

Thank you everyone. We will be in Barcelona (our 4th visit) and will be flying out of Barcelona too. Just wanted to see more of Catalonia in a very short time. I like the idea of going north. Montserrat sounded very interesting to me, also. I am intrigued regarding Vilanova i la Geltru. If you have another idea, please let me know.

Since you will have a car you will have the flexibility of visiting more off the beaten path areas. Here are some places of interest I would recommend south of Barcelona:

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.

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.

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 that led to the Second Punic War. Now an eclectic mix of Roman, Moorish, and Christian fortifications.

Valencia - 3rd largest city in Spain, hometown of the Paella. Known for its futuristic structures of its City of Arts and Sciences. You will find also Modernista architecture, great museums, and long stretches of beaches.

Albarracín - voted the most beautiful village in Spain, picturesque Moorish fortifications surround this medieval pink-hued village. Was once the capital of it's Berber taifa kingdom, currently in the works of becoming a UNESCO World Heritage Site. A good option for an inland day trip from Valencia.

Palmeral de Elche - one of the largest historic palm groves in the world, dating back to the Moorish times, designated a UNESCO Site. The Palm forest is surrounded by the town of Elche, one of the oldest in Spain, with much Iberian and Hellenistic archaeology.

Of course, you may not have enough time to see all these places, so you will have to pick and choose depending on your interests. I have not been south of Cartagena (good Roman ruins there too), but I have heard that the coastline of Cabo de Gata National Park is quite impressive.

You have all been of so much help. I know we will have a fantastic trip. THANKYOU