Part of the country we have no idea about. We have all day Saturday (pickup hire car at 0900 and drop back off at 1600 at the very latest). So probably max of about 2.25 hours out from Cardiff.
So where to go? We like the coast, we avoid motorways. A quick google suggests somewhere like The Mumbles might be interesting or Rhosilly Bay perhaps with a stop in Porth Cawl - but we know nothing of this part of the country.
Pendine Sands is (obviously) on the coast and has a very interesting automotive history, but it’s really quite a hike from Cardiff. You’d spend most of the day in the car.
If you had a bit more time I would say head for Tenby / Pendine and then drive back along the coast via Saundersfoort and Laugharne but I agree its too far.
Porthcawl itself is a dump. Think Canvey Island or Margate.
If you want to avoid motorways and be near the sea then the Vale of Glamorgan is probably your best bet. You can use A roads from Cardiff, go via Cowbridge perhaps and come back via Llantwit Major. There are some ancient pubs and an ancient church in Llantwit
Head for Southerndown beach and Nash point / lighthouse. You could do both I would think.
Lunch -
or
This blog is very good and he has reviewed the other places where you are going to eat.
Made it to Rhossili by motorway - the timing was just perfect without being rushed - we pickup up the hire car at about 09:20 and got it back to Enterprise for 15:50. Had enough time to drive to Rhossili and back, have a decent walk and some lunch.
Many years ago I met Stephen Terry and he was a thoroughly nice chap. We also knew the Sommelier from when he opened the Hardwick, but we have never managed to get there…