Southerndown - Glamorgan
A good sandy beach with plenty of rockpools. A walled garden is nearby, and walks to Temple Bay and Southerndown village and pub.
Merthyr Mawr - Bridgend
A quiet sandy beach, about ¾ mile walk across the dunes of Merthyr Warren. One castle by the car park, and another a short walk away from Merthyr Mawr. Both supposedly haunted!
Three Cliffs Bay - Gower
A wide, sandy beach backed by a valley and flanked by limestone cliffs. Plenty to explore and lifeguards in summer.
Pembrey Beach - Carmarthenshire
A 7½ mile long sandy beach backed by Pembrey Country Park and Pembrey Forest. Relatively warm sea, lifeguards, barbecue areas and generally safe swimming. Plenty to do. Bring a bike if you want to explore the 4 square mile forest.
Priory Bay, Caldey Island
Arrive by boat from Tenby (about 25 minutes crossing time). Explore Caldey village or walk the path around the island. At low tide you can walk along the shore to Sandy Bay.
Broadhaven South - Pembrokeshire
A wide sandy beach - freshwater stream and large pools on one side, freshwater springs on the other. Backed by Bosherston Lily Ponds. Good clifftop walks along a spectacular coastline. Pub and cafe in Bosherston
Barmouth - Gwynedd
All you'd expect from a seaside resort. Good sandy beach, lots of facilities, walk / cycle across the ½ mile long Barmouth bridge and explore the Mawddach trail, or climb Dinas Olau hill behind the town. Does have traffic problems though - come by train if you can!
Newborough Beach - Anglesey
A long sandy beach backed by dunes and Newborough Forest. Plenty of forest trails (including one for runners). Nearby Llanddwyn Island is worth exploring and can usually be reached without getting wet, as can neighbouring Malltraeth Sands.